Stick it in her ass.

You slide your cock right out of her pussy and she gasps a little and whimpers.

"Why'd you stop?" Kelly asks with a whine. "Stick it back in and pound me, Master."

You smack her ass and she yelps. You smirk and stay quiet as you bring your cock up and start tickling her puckered hole with the head of your slick dick.

"Oh, fuck, Master," your sister moans. "Don't, don't. You're too big. I've only done anal once. I can't---"

"Please stop whining, my slut-sister," you say in a commanding voice. You caress your smooth, round ass with one hand as you use the other to keep circling her asshole with the head of your cock.

She starts whimpering a little. You see her butthole clench tightly. You chuckle and push your cock against it harder.

"You were begging for anal a few minutes ago. What changed?"

She pants hard and answers. "There's a difference between craving it and getting it. I... I just don't know if I'm ready for an actual big cock up my ass."

"Kelly..." you say, then stop. You were going to be a little nice, but you really want some anal. Thinking about Kelly's usual bitchy nature, you decide... Fuck it. Literally and figuratively. "Kelly, please unclench your hole and beg me to fuck you up your ass. Please make your asshole sound as inviting as possible."

She shudders hard as her tight hole unclenches a bit. "Please stick your dick into my ass, Master. My ass is the tightest hole on my body and it's begging to have your big, hard piece of manmeat inside of it. God, I really want you to shove it right into my warm asshole and thrust. Please stick it in---" She gasps and moans as you start to shove your dick into her little starfish. It's tight, just as tight as it was the other night. And you love it. You slowly work your cock into her until you're fully sheathed inside her bubble butt.

"How does it feel, Kells?"

"I feel so fucking full in my ass, Master. It's a little painful, but it's more pleasurable than anything. It's so satisfying to have my ass full of your cock."

You moan and start thrusting your cock, slowly at first, but going at a steady pace in no time. You bring your hands down and squeeze her cheeks hard, spreading them and her hole at the same time.

"Oh, fuck, Master," she says. "Your cock is a fucking monster. I can't believe how big you feel inside of me."

You bite your lip and give her ass a smack with both hands. She yelps and moans. "Smack my ass, master. Spank me. I love it."

This encouragement is all you need to really start going to town. You fuck her a little faster as you smack her with every thrust. You can almost feel the jiggle of your ass go into your ass and jiggle your cock.

"You're such an anal sister-slut, Kelly. You're going to be my submissive cum **** for the rest of your life." You say, your throat hoarse with pleasure.

She moans and gasps with each thrust, each spank, each word from your mouth. "Yes, Master. My holes are yours to fill with your cum whenever you want. Please use and **** me whenever you wish." She groans. "Oh, fuck, I shouldn't be saying that to my own fucking brother. You're such a damn twerp, Master, but you're so good with your big dick. Keep fucking me, Master, I'm almost... I'm gonna..." Her words are lost in a gibbering mess as she cums with your cock in her ass, squeezing your dick tighter than you've ever felt.

You fuck her fast now, her warm, tight hole sending jolt after jolt of pleasure up your shaft. You're almost there. You could cum any moment. But where? Do you give her an anal creampie? Shove it back in her pussy? Make her take your cum down her throat? Something else?