"I sure hope this works..."
A nervous young Labrador by the name of David clutched a tiny bottle in his uneasy paws, trying his hardest not to drop it as he tiptoed his way down the hallway, towards his sister’s room. An inch a drop shall make them even, it read, and though David knew what effect he hoped the potion would have on his sister, there was no way that he could be entirely sure until he tried it on her. Having gotten the vial from a rather sketchy old man running a curio stand, he had all the reason in the world to be nervous, and yet, his excitement kept him from listening to his better judgment.
Creeeeeeeeak…the aging wood floors whimpered under David’s footpaws, and he silently cursed the house, wishing that it would have picked a different time to settle. He simply wasn’t sure if Emily, his slightly younger sister, would have agreed to this, so giving it a try when she was resting seemed to be the best bet. “That way, if she ends up not liking it, I can always just go back to bed and leave her in peace…” he reasoned quietly, as he lifted a paw to delicately push Emily’s door open. 
Waiting for another creak, David paused, but to his relief, no further sounds came. The door easily swung open without a squeak or a creak, and the young Labrador male started to make his way across his sister’s room. It was a simple room, much like his own, but with a few more feminine accents; the walls had a paint of pink instead of simple white, and there was carpet upon the floor instead of hardwood, something that David was suddenly very thankful for.
The mischievous lab kept tiptoeing across the floor, pawpads clinging to the carpet as he held on tightly to the potion. It had cost him a pretty penny, and he wasn’t about to spill it, unless it was right upon Emily’s body, just as he’d been instructed. 
Within a few short moments that felt like lifetimes to David, he was just a breath away from Emily, standing over her as she slept peacefully, body entangled in a blanket made of soft, downy feathers. It seemed almost a pity to David to wake her from such a deep sleep, and his nerves were keeping him from asking anyway.
Pop! The lid of the vial popped off right between David’s thumbs, thankfully not spilling even the tiniest drop as he readied the potion. The glass was shaking between eager pawtips as David hovered over his sister, knowing he just had to land a single drop upon her fur. Taking his sweet time, David stretched his arm out over Emily’s body, making sure not to spill on the blanket…not a drop could be wasted. 
Time froze the moment that David tilted the bottle, and one clear, nearly invisible drop started to spill from the vial. David could nearly see the bubbles of air bursting in the clear liquid as it teemed down from the vial and landed with an inaudible spat upon Emily’s collarbone. At first, the female labrador didn’t so much as stir from the potion.
David flattened his ears down to his head as he placed the top back on the bottle. Though her nether were covered by the blanket, it seemed that the potion was having no effect on Emily whatsoever. However, determination was starting to overwhelm nerves, and David was not going to be denied an answer. Hastily, the labrador male set the vial on his sister’s nightstand and walked back over to the bed, grasping the cloudy-soft covers in his paws. With a quick, careful tug, David was able to pull the covers back just enough to completely expose Emily, literally and figuratively, he found…his sister decided to sleep in the buff. 
That’s convenient, David thought.
The opportunity was far too much for David to pass up. His intentions in this nighttime rendezvous were already a little less than pure, and as such, nothing was going to stop him from admiring his sleeping sister. He’d hidden his love for her from the public eye, and from her own for so long that it nearly pained him. His heart ached anytime that another male came close to her, or spoke words of love for her. He could feel a slight wrenching in his gut and butterflies tickling all the way up his tummy whenever Emily would steal a hug from him, or plant a friendly kiss on his cheek. The feeling of her breasts pressed up against his chest, even through two layers of clothing was enough to completely set his soul ablaze.
Now, he could see those same soft mounds, resting so perfectly upon his sister’s frame, and it took every bit of his willpower not to give them a gentle squeeze. His pawtips trembled at the very thought, but his mind managed to keep control for the time being. David’s eyes wandered freely, watching the smooth, dark fur upon Emily’s bodice start to fade and thin as it drew down towards her uncovered snatch…and just above it, just like the man who sold the potion had said it would…a small growth was starting to form, just a hair above the bright pink clit that stood stark against the dark fur surrounding it. 
The growth was formless, at first; it appeared to be nothing more than a worm trying to wriggle free from the surface of Emily’s skin. But as the seconds passed, each one feeling like an eternity to David, the growth started to take shape. The base thickened and became clearer, sticking out more fully from the flesh it grew from, and a head was taking shape quickly at the tip. Oddly enough, it seemed that the mystical appendage was already circumcised, something well beyond David’s understanding. The potion seemed to make the freshly grown member have a mind of its own, as the flesh was already starting to bulge and grow stiff, without any prompting. And then…
Emily winced.
An adorable little whimper escaped her pursed lips, and her arms came back in towards her body as a tingle ran through her unconscious form. David had been silently hoping his sister would stay asleep and allow him a chance to get familiar with the new length, but fortune was no longer in his favor, and blurry as the vision given to her was, Emily’s eyes started to open.
“D…David…? What on earth…?!”
Emily squeaked awake, her voice little more than a whisper at first, but her lungs were nearly at their height as she brought her paws up to her eyes and rubbed them furiously, still too tired to clearly make out what was going on, but she could feel an alien tingling in her groin, and she could see something that definitely didn’t belong above her netherlips.
“Shhh! Emily…you have to keep quiet! If we’re not careful, we’ll wake mom up!” David whispered harshly, trying to keep his own voice down, and trying even harder to keep his cool. He’d been caught red-handed, and at this point, there was no use in trying to deny his actions…his only option was to try and explain them.
Emily managed to finally knock the sleep out of her eyes enough to get a clear look at what was going on. She was naked as the day she was born, and that was the most normal part of what was happening. She slept the same way every night, so she’d expected that, but she’d never expected to see David halfway between her legs, and never could have even dreamed that she would have grown a cock. 
“David…if you have something to do with this, you better start explaining right now, or I’m going to tell mom that you snuck in here!” Emily replied in kind, giving her brother a stern, displeased look, narrowing her eyes upon him in the darkness. She immediately crossed her arms over her breasts, having only just really remembered that she was topless, in front of her brother, no less.
David had just enough nerve to sneak into Emily’s room and pour the potion over her body, but now that he had been put on the spot to explain himself, he wasn’t sure what to say. Despite his best laid plans, he hadn’t anticipated Emily actually waking from her slumber…the man at the curio store didn’t mention anything about that. “I…I…listen, Emily…I just…wanted to…try something with you…”
The stammered words did very little to appease Emily. “Try something with me? What do you even mean? And why couldn’t you just wake me up…?”
“Because!” David quickly interjected, wincing his eyes closed as a fresh wave of bashfulness overtook him. He had no idea how loud he’d been when he yelled, but Emily’s aggravation immediately melted, replaced with a look more of concern and surprise. “I...I just…damnit, Emily…I like you! I could never tell you before…and I could never tell you when you were awake…I just couldn’t get up the nerve, and there were so many things I wanted to do with you, and I was just so worried about how you’d react that-“
“Easy, David…slow down,” Emily interrupted this time. Though she was trying to appear as in control as she could, a thin layer of pink was nearly visible under her cheeks, even in the dark of night. “I had no idea that you liked me…I wish you would have said something sooner, but, what did you do to me? What did you want to try with me?”
The male labrador was a bit stunned by his sister’s comforting words. “You wish I would have told you? Really…?” he asked, a bit bewildered by the response, so much that he almost forgot to explain himself further. “Oh, that…I…well, I may have gotten a potion that changes your body so that you can be more like me…I thought it could be fun to, y’know…mate with my sister, and then let her do the same to me…”
David’s voice started to fade near the end, but Emily pricked her ears forward and caught every word. She was completely shocked at her brother’s confession, and put on the spot as she was, part of her wished that she had somehow stayed asleep, but now, she had no choice but to be honest, as well. “I won’t deny that I do have an awfully cute lab for a brother…I’ve thought about it too, honestly,” she admitted, glancing down at the bed, but still getting an eyeful of the new member she’d been graced with. “But, David…I wouldn’t even know what to do with it. I don’t really know what having one of these is like. How does it work…?”
“I could always show you, if you’d like,” David offered. He still felt almost as shy as his sister looked, but as she seemed to grow comfortable with the situation she’d been thrust into, he relaxed as well. “I mean, if you…would like to mate with me…”
Emily was hesitant, to say the very least. She did have a very cute brother, in her eyes…perhaps even gorgeous, but this was all happening to her so suddenly. She scarcely had time to process everything that was going on as it happened, and as she tried to shake the cobwebs of sleepiness out of her mind, she couldn’t help wondering if maybe this was all just a dream…and that maybe she should act like it was.
“Maybe, just this once, David…we could try it. I’m wide awake now, there’s certainly no point in me just going back to sleep. Maybe we could have a little fun with it…”
Simple as her words were, they were the best words that David could ever have heard at that very moment. Forgetting his better judgment, the male labrador tore himself out of his PJ bottoms, leaving them carelessly on the carpet. “Thank you, thank you, thank you, Emily! I promise you’ll enjoy this…you won’t be sorry…”
David clamored into the bed as quickly as he could, having done nothing to hide the fact that his own length was as stiff as Emily’s magical endowment. To his interest, they looked to be nearly the same size, and awfully similar in appearance. David was starting to wonder if Emily had seen him naked before, and perhaps, the potion took on the looks of whatever the person it was used upon could best imagine. 
“Shh…” Emily shushed David, reaching out a paw and pressing a pawtip to his lips. “Remember, we don’t want to wake mother. Now then, dear brother, show me just how you would use one of these,” and on that, she slipped her pawtip down from her brother’s lips, teasing the very tip along his cheek, along the side of his throat, and right between the middle line of his chest and abdomen. It seemed as though she might stop there, and David trembled under her touch, but she didn’t keep him waiting long, and only seconds later, she reached back and grabbed the magical length that had grown from her body, squeezing it playfully in her hands and providing David with a sight to make his own member throb with desire.
“Are you sure that you’re ready? I don’t want to hurt you…” David admitted, not bragging himself up at all, but just making sure to take care of his sister. To say he was inexperienced was untrue, but to say that he was still a little unsure of his skills was more to the point. “It’ll only be for a little bit…”
Emily smiled up at her shy brother, flickering her tail underneath his rump and brushing it along his sack to urge him forward. Emily wasn’t the most experienced lab either, but she knew enough, and fantasies ran through her mind just as much as any healthy young woman, sometimes, even about the very brother she was set to make love with. “I’m sure, David. Just be gentle with me, and show me what you’d like me to do to you…”
David had been so excited to get to this moment that he’d almost forgotten how nervous he was, but those nerves were starting to creep back up on him now as he leaned over his sister’s gorgeous, naked form. The closer he got, the greater warmth he could feel coming from the alluring valley of her slit, and as the very tip of his stiffened cock touched to her outer lips, he felt that he might faint from the thrill. “It’s easy, Emily…I can feel that you’re nice and wet for me already, so when the girl is ready, you just push a little bit, like this…nngh!” David gasped, feeling the whole of his head penetrating his nearly virgin tight sister. “And then, when you can feel the inside muscles adapting to you, you push a little further forward, until all of it is inside…”
The rest of David’s length started to slowly disappear from his sight, slipping into his sister and finding himself right at home; Emily was a perfect fit for his cock, and her body shuddered with pleasure at the feeling of being so perfectly full. “That…oooh! That’s wonderful, brother…I know you said it wouldn’t be long, but please, don’t stop yet…this feels so nice!”
“If you insist, sis,” David said, grinning blissfully at the pleasure derived from their incestuous mating. As much as Emily was still pretending to be in a dream, David felt like he really was in one now; Emily wasn’t just okay with what he had done, but she was turned on by it, and now, the young brother and sister were sharing their first sexual experience in a way that David had only ever been able to imagine. “We just have to be careful that I don’t get too excited…if I leak any seed inside of you, that could be really bad…”

“Ahhnn! Mmm…don’t worry about that, David…just worry about how good it feels to be plowing your sister’s juicy cunt,” Emily retorted amid a sudden, loud moan, starting to sink further and further into her awoken dream. Though she knew full well by now that she was awake and truly having sex with her brother, the idea had taken root in her mind, and she found herself aroused beyond reason by it. Even the cock that had grown from just above her quivering clit was starting to seem more at home, as she wondered just how she was going to use it on David.
“It feels incredible, sis…so much better than I ever dreamed!” David tried to keep himself quiet, but his voice climbed into a yelling moan, unable to help himself, and he was starting to care less and less about the possibility of getting caught, far too caught up in the heat of the moment to be bothered with worry. “Just remember that you still have to return the favor, after all.”
Emily narrowed her eyes at David amidst a devious grin. She leaned her body up, wrapping her arms around her brother’s back and holding her soft, supple breasts to his chest, all the while looking right into his eyes with a stare that could melt a lesser man. “And just how am I going to do that, hmm…?” she asked, but before she’d get her response, she stole a sudden, firm kiss from David’s lips, nipping her fangs right into the tender flesh about his muzzle.
David was so lost in the kiss, so swimming in pleasure that Emily’s words landing upon his ears were the only thing that could bring him back to a rational thought. “I…” he pondered aloud, slowing his thrusts until they came to a complete stop. “I guess I hadn’t thought quite that far ahead, but I know the old man said the potion might do something for me, too…maybe I should try it?”
Emily reached back and grasped the potion hastily off of the nightstand. A smoldering glance of desire played about her expression, and it was clear that now that she’d drank from the challis of pleasure, she wasn’t going to stop until she was drunk on it. “Go for it, and please, don’t keep me waiting…” she rumbled, a low growl building in the back of her throat as she watched David eagerly.
“I sure hope this works…” David whispered, clasping the bottle in his paws and popping the top once again. Unsure of the best way to use it, and what might be the fastest, David tilted his head back and poured a tiny amount, just barely more than what he used on Emily, right down his throat. 
At first, he felt nothing.
Moments later, he felt completely empty.
David’s eyes widened in fear as he glanced down, but still hilted inside his sister, he couldn’t see anything. It wasn’t until he withdrew his slick, juice-coated cock that he could see his sack starting to shift a little bit, and just behind it, he could feel his flesh changing…opening. Emily grew curious, and looked down between David’s legs to see what was happening, and she couldn’t help a playful smile as she caught sight of the forming of two already dripping lips, forming cute labia, extremely similar to her own. “Sis…what’s it look like?” David asked, still looking a bit harrowed by the experience. 
“It looks just like mine! How about that…?” Emily asked, reaching forth a single pawtip to dip it inside, seeing how moistened David’s own magical endowment already was. She slipped in easily, even as virgin as David’s body was. “And it’s already ready for me to try it…David?”
“Yes, sis?”
“Do you…mind if I cum inside of you?” Emily asked hesitantly, looking up at her brother with that unfairly pleading look that she used whenever she was determined to get her way with him. 
David could never form a proper response to that teasing little look, and so he simply shook his head.
“Good. Then bend over so I can take this thing for a spin,” the female labrador commanded, smiling playfully as she knocked her brother back onto his rump, and made a quick spinning motion in the air with her pawtip. David readily obliged, turning around and resting himself upon his paws and knees, just as he lifted his tail to reveal a clean kept tailhole, an enchanted slit, and the back of his sack. To Emily, it all looked a little unnatural that way, but in a way, nothing about this was normal to her when it had started, and that was just fine by her.
David gripped the sheets nervously as he felt Emily climb over his body and rest her paws on top of his, but the female lab gave a reassuring kiss to the back of her brother’s ears, and did her best to adjust to her newfound member. It was a bit awkward for her to line everything up right at first, but with each try, Emily’s cock got a fresh splash of arousal from David’s slit, and with each pass, a quiet hymn of moans filled the air around them. With their proper sex before already leaving each sibling hungry for an orgasm, their enchanted sex would be the perfect way to cap it off.
“Don’t worry about cumming too soon, brother dearest. Girls can get off more than once at a time, so maybe you’ll be able to do that now, too,” Emily suggested. David was going to nod and reply, but just as he’d settled in to his new position, Emily speared him on every last inch of her new length, robbing him of his voice, replaced only with a near silent squeak of surprise as Emily hilted her brother perfectly. “Ooooh! David…that feels amazing…so that’s what it’s like to have a cock…”
Emily’s moans were likely too loud to be safe, but David could hardly even moan as he adjusted to the sensation of being filled this way. “Emily…do…do that again!” he begged, finally finding his voice again, and finding himself completely overwhelmed with pleasure as the girth of Emily’s cock fit almost too perfectly inside of him. Just to put an exclamation point on his begging, David leaned forward and bucked his rump back at his sister, only able to hope that she would give him exactly what he wanted. “This feels so amazing…I can’t get used to this…I’m gonna cum any minute, Emily!”
Emily grinned brightly, her fangs shimmering in the moonlight as she released one of David’s paws and took a grip at his neck scruff instead, quickly getting into the role of a dominant male that was plowing a heated female. “Then go ahead and cum, David…I won’t cum until I’m good and ready. Not that it’ll be too long before then,” she admitted, having to grit her fangs to keep from moaning any louder than she already was. With each thrust, the slap of her hips against her brother’s pert rump filled the room, and the house did little to keep the noise down, but as far gone as they were, nothing was going to keep them from finishing.
“Just don’t hold out on me, Emily…this is so much better than I ever imagined…please, fuck me sister!” David continued to plead with Emily the best that he could, but the new inner walls of his pussy started to flutter, and a new sensation overwhelmed him completely. His first ever female orgasm rocked his body, filling him with a new kind of pleasure all the way from his head to his footpaws, the pads of which curled tightly into his own flesh. “Holy fuck…Emily! I don’t…I think…I just came!”
“Ride it out, David. It feels great, doesn’t it? Sure is nice being a female…but being a male isn’t too bad, either! Feels like my tummy is all tied up in knots…I think I’m gonna cum too, David…try and squeeze me as tight as you can!”
Emily couldn’t keep her abdomen from tensing up, and the pleasure grew from her groin, flowing throughout her body until she could feel something coursing through her newly formed cock. Though she had grown no sack, hot, sticky seed spurted forth from the tip of her enchanted cock and filled David so full that the white, thick cream spilled back down over his thighs and right down onto the long forgotten blankets, leaving them stained with a mix of incestuous arousal. “Ooooh…damn! Who knew having a cock could feel this good? David…I’m so glad you talked me into this…fuck…fuck!”
David knew that his mother had to be jolted awake by now, but he was so lost in pleasure that he could care less if his mother walked in the room right then. He was as close to his sister as he could be, and he wanted nothing more than to feel her enchanted cum filling the passage that he shouldn’t even be able to have. “I would have warned you that it gets pretty intense, but you literally took my breath away, sis…that was…unbelievable…”
David’s words began to slur as he spoke, his voice little more than a string of moans that grew softer as he spoke. The second side effect of the potion was starting to take its toll as David’s eyelids grew too heavy to hold open, and Emily felt the same sensation. Her maw opened wide with an exhausted yawn, and still resting atop her brother, she started to drift back into a peaceful sleep, still buried inside her incestuous mate, who was joining her in sleep only seconds later.
**
The pleasant sound of birds chirping along the window sill brought the embrace of morning around David and Emily, who had grown so exhausted from their sexual endeavors that they passed out almost immediately afterwards. They had no idea that it was just the harsh side effect that the potion had on their bodies, but as Emily fluttered her eyes open, she could immediately recall everything that had happened the night before.
“…Could it be…?” she wondered aloud, noting that she’d passed out right on top of her brother. She reached a curious paw down between her legs, but felt only her slit, right where it was supposed to be. A little further reaching revealed to her that David was definitely only a male again, having no trace of his enchanted netherlips, and there were no stains from the seed that Emily had spilled deep inside his body.
“I think so,” David said, as he turned his head back and looked up at his sister through half-lidded eyes. “I guess that potion worked after all, even if it took the wind out of our sails…”
Emily nodded her agreement as she slipped off of her brother, letting him sit up next to her if he so pleased. “I was beginning to wonder if it was all just a dream, but…I guess it really happened. I’m sorry if I forced you into something you didn’t want, David…”
The young male lab shook his head with a smile. “I brought it up to you, Emily. I’m just glad you enjoyed it as much as I did, at least, I hope you did…”
“I definitely did, maybe a little more than I realized,” Emily admitted. “To be honest, I had so much fun having a cock that I was hoping I’d still have one in the morning.”
“Well…”
“What?”
“I still have plenty of that potion left, you know,” David said, as he pointed to the mostly full vial on Emily’s nightstand. “There’s no reason we can’t do this again sometime, that is, if you’d like to…?”
Emily looked down for a moment in pondering, before turning back to kiss David on the cheek with a cheerful smile. “I’d love to, but next time…let’s try not to wake half the neighborhood, okay?”
David snickered. “Maybe we should just worry about waking mom, first.”