She tells you to start by filling up her ass again

"Alright, Doug, Since you came before me, I'm going to need you to finish fucking my ass before we can continue with anything else."

"But, Crystal, I already came... I can't go again, not so soon," you protest meekly. Crystal smiles down at you, pulls her cock away from you, and steps back toward the bed.

"I know." she replies, "and your dick isn't as big as my vibrator anyway, so it really wasn't cutting it for me. No, you're going to fuck me with something bigger. Now get over here." She sits down on the bed and scoots back a little, then lies down on her back, cock still at full attention. You stare at the huge thing for a moment, bowed over so much with its own weight that it rests firmly between the bottom of her sizable and firm breasts. She lifts her legs up and pulls them back, exposing her tight little butthole and pussy hiding under her huge balls. "Doug, come here and give me your hand," she says, and you reluctantly do as she asks. She takes your hand and pulls it to her mouth, sucking on your fingers, gagging on them. When she decides they're sufficiently lubed up, she pulls your hand out of your mouth and moves it down to her ass. "Put it in" she says.

"What? But-but... Are you sure it'll fit?" you ask, in shock.

"Of course I'm sure. Now either stick it in me or turn around so I can stick something in you." You hurriedly start pushing in your fingers, one at a time. "Oh fuck no, Doug, give me the whole thing!" she yells. You quickly oblige, pushing your whole hand in as far as it will go. You make it all the way to the bottom of your thumb before you encounter any real resistance, but Crystal doesn't seem to mind. She pushes against you, and you push back until your hand slides in. The **** of your push carries through, and your fist keeps going into her ass until your forearm is almost completely engulfed. It's It feels very strange, having your fist in her ass. It's squishy, but at the same time, you can feel the hard strength of her muscles as she contracts around your arm.

Crystal's head rolls back in ecstasy and she starts to moan. You're pretty confident that she's totally unaware of anything other than your fist and the feelings it's making. You pull out until your wrist catches on her sphincter, then start to push back in. Suddenly, you stop. You've noticed something strange in her ass. It feels spongy and large, and when you touch it, she immediately increases the volume of her moans and clenches hard around your wrist, almost cutting off circulation. This must be her prostate! You've heard about this of course, both in Sex Ed. and on the internet, but you never imagined it would be this big. It's almost the size of a baseball. You open your hand as much as possible, spreading your palm over it and caressing it. Crystal certainly seems to like it, as she starts to furiously jerk her cock with one hand and maul her breasts with the other.

You continue to play with her prostate for a little longer, until her ass clenches tighter than you've felt so far, cutting off feeling in your hand. The clenches increase in frequency until Crystal cums. Even though you knew it would be big, you're blown away by how much cum erupts from her cock. The orgasm seems to go on forever, covering you with wave after wave of your sister's cum. Finally, her orgasm subsides and she lays on the bed, gasping for air. You start to pull your hand out, and she twitches violently at any movement, but you finally manage to pull out completely. As you stand to go, Crystal grabs your other wrist, preventing you from leaving.