She tells Sweetie, and Sweetie is okay w/ it. Then starts suckling at Rara’s teats.

“I was just curious what it was like... I always only had a bottle and um...”

“Because I didn’t want you to... know.”

So Rarity is like okay fine. And then all despite the crude nature of the gesture, it was honest and relieving, and in fact it was just the sort of thing Rarity never even knew she needed to feel.

Then Sweetie’s all, “Ha! You <i>do</i> still have some!”

Rara has to teach Sweetie that pony teats produce milk from stimulation, only requiring pregnancy to mature. Thus introducing a whoooooole new fetish for her little sister to try.