>Before penetration you slowly run your hand down her hot, wet back
>She shivers with your hands presence 
>You start to lick up and down her back, letting your cock tease her already wet pussy
>She keeps shivering with every inch you move
>The warm water has dampened her hair, which is now covering her face
>You couldn't tell this but her eyes were clenched shut, anticipating the impending sensations about to come
>Your cock was digging in to the bottom of her slit with some force
>Besides the shower water, you can feel yourself getting wet with a different liquid
>Enough with the games
>You rear back and thrust in about half of your length
>The sudden thrust did all but make Rainbow Dash black out
>She was still, so tight
>Her natural lube was doing wonders to your cock
>Her warm insides we're massaging the giant intruder
>You can hear her moans over the shower 
>Damn, hearing your sister make those noises excite you even more
>You shift your weight back a bit and rest your hands on her back
>You decide to be nice and work slow at first
>You thrust back in, to about the same length
>"Ohhhh"
>You give her a few seconds to get used to the thick shaft before pulling out a bit more
>You really wanna go to work in her ass, but she said no
>You shift back in, this time a little deeper
>She squeals at the thick cock going deeper
>It almost seems as if it was her first time again
>You slowly caress her tits with each hand as you pull back out
>You squeeze them tight as you push back into her, this time almost to the hilt
>Her pussy clenches up and her head shoots up towards her back
>That must of been a deep one
>But you still have an inch or too left to go
>You are still feeling up her tits, not very big, but still firm and perky
>You didn't notice at first but she was speaking
>"What...are you waiting...for?"
>You push her head as far as it can go to the side, so you can see her face
"I wanted to see your expression when I do this"
>You lean your weight back and in one thrust you push yourself all the way into her
>As deep as you can go
>Like the other times, you feel a hard resistance against your head
>You hear what almost sounds like yell of pain
>Looking down at her face, you see all types of emotion
>Pain, pleasure, relief, desire
>Her mouth is dropped open, and her eyes, clenched shut
>Her wet hair almost covering her whole face
>Her breathing is shallow
>Her deepest muscles, milking your shaft with every spasm
>Her head falls back down into place, no longer looking back at you
>You decide to try not to hit her in the cervix anymore
>You do care about her
>You pull out, feeling her pulling you, trying to keep you in
>After pulling out push back in, this time, stopping about half an inch short of her cervix
>"Th-thanks"
>You smirk to yourself
>You can feel her muscles quiver and ache around you
>The feeling is toxic
>Your whole body feels the pleasure
>You lean back out and thrust back in
>You can see the pleasure shoot all through her body
>Her pussy is twitching and constricting your thick shaft
>Still inside of her, you begin to massage her ass
>Squeezing and smacking each cheek
>Each smack gives you a soft whimper from in front of you
>You pull back out and thrust in again, this time faster
>Each thrust is increasing in speed and violence
>You can feel your head slam against her cervix, even though you were trying not to
>Her moans of painful pleasure fill the shower
>You can tell she's had a number of climaxes, but you are to busy fucking through them to notice
>You continue this for 10 more minutes
>You notice that her legs are shaking
>Probably because of the pace you were working at 
>You turn her around and place he on the floor, sitting in an obedient position
>Water splashing on her face
>You sit down as well
>You push her onto her back gently before climbing over her
"You looked like you were getting tired, so I thought you could lay down"
>Your cock still pressing up against her soft little mound
>You lean forward, placing your hands on either side of her neck
>You lean your head down to give her a kiss
>She wraps her arms around you and kisses you
>Your tongue is caressing hers, licking the inside of her mouth
>While still hooking up, you lean your groin back and thrust into her
>You feel her whimper through your mouth
>You pull back from the kiss and thrust a little further in
>And then further
>Until you reach her limit again
>You lean your crotch back and thrust in again, just missing her cervix
>She shutters with each thrust you take, hoping not to get hit with extreme pain
>You feel the water running down your back as you nail your sister
>The moaning becomes faint as you pick up your pace
>You can feel yourself getting close
>She is holding on to your neck now, trying to pull your head in for a kiss
>You happily agree and lean in for the kiss
>Unfortunately, you lose track of your thrusts and hit her in the cervix...quite a few times
>Her body tenses up, feeling you so deep inside of her
>Her muscles have stopped twitching, but still has enough tightness to add MORE than enough pleasure
>The orgasm is growing stronger in your loins
>You hear her moan as she climaxes one final time
>As you feel your climax coming, you don't slow down... In fact you speed up
>Slamming her wall over and over again, causing her to wince in pain with every thrust 
>Oh god
>It's coming
>You let out a load moan as you thrust in as deep as you can and release yourself
>Your semen coating her inner walls, building up more and more pressure with each spurt
>Her tired gasps for air increases the sensations
>Burst after burst your semen builds up deep into her gushing cunt
>Her head falls back as you slowly pull out of her abused pussy
>Her breathing is very erratic and shallow
>You smile as you see your cum slowly run out of her cunt, along with her own fluids
>You give her a soft kiss on the head before standing up over her
>You give her a hand to get up
>Slowly and weakly she pulls herself up, with your help
>"That was... the best fucking...you have ever done"
>You just smile as you let the water rinse off your sweat and smell from your body
>She still seems dazed and out of it
>She'll be fine
>At least you were able to cum inside Rainbow Dash