Live up to your deal and fuck your sisters ass.

Partly because you were horny as hell from watching your mom inadvertently teasing you, partly because Kelly's asshole was glorious, but mostly because you feared for your life if you didn't comply, you let yourself be dragged out of the kitchen by your single-minded sister. "Brb Mom! Gotta help Kelly with something! Don't go upstairs or listen to us till we come back!" You had your fingers in the MCR for that last bit, unwilling to upset the uneasy balance you'd been able to give your mother. "Oh. Ok, but breakfast is almost ready!"

"Keep it war-BANG."

Kelly is on you the instant the door shuts, shoving you up against it without a care for the boyband poster that tears free from its tacks. Her eyes are wild and her breath comes in ragged gasps, "I can't take it anymore John, I need your hard cock in my tight little asshole! Right. Fucking. Now!" She falls to her knees, scrabbling clumsily with your pants but having them down in record time, most of your cock vanishing into her mouth.

"Jesus Fucking Christ! Kelly! Oh fuck!" You're simultaneously the most frightened and turned on by your sister as you've ever been. With the way she's going you're amazed she doesn't graze you with her teeth, "Fuck Kelly, you're such an amazing cocksucker!" She bobs her head even more aggressively, your cockhead slipping into her throat as she wets your cock as speedily as she physically can.

As quickly as she was on you, she's off, bending herself over the bed and sliding her yoga pants down to her knees, arching her back, "Fuck me, fuck me, fuck meee!" Thank God you'd told your mother not to listen, you're a little nervous the neighbors might have heard that. "Take it easy Kelly, holy crap! We have time today, we can take it slow! We could even do some other things if you like-"

"NOnono-John! I don't want to do anything with you, I need you in my ass, fucking me! I'm not aroused by you, not attracted to you, not into you in any way, I need your cock! In. My. Ass. Please!" You always thought the whole 'crazy chicks are hot' thing was overblown but you had to admit: even if you had no idea what was wrong with your sister you'd still want to tap that. Grumbling something about ungrateful nymphos, you tune her pleading out for a moment as you retrieve a bottle of lube from your room.

"Why are you not in my ass already!? I lubed you up, I don't care if it's a li-AAh!" You silence her with a smack to her exposed ass, popping the lube open and sticking its mouth into her rosebud. She squealed as you unloaded the remainder of the quarter full bottle of cool goo into her insides, "This isn't about you Kelly. You want me to last a while this time don't you? Well I don't want my dick rubbed raw by your tiny ass. Now pucker up buttercup." You thrust forward, doing your best to take it slow yet finding yourself nearly hilted moments later.

Kelly's moan almost escalates into a scream before she bites down on the bed sheets and muffles herself. You're not guiltless for the racket either, groaning as the lube and her squeezing ass do ungodly pleasant things to your iron-hard erection. Yesterday was pretty great, even setting aside the excitement of the first time, the friction was hard to beat. This gave it a run for its money while somehow being completely different.

If anything she feels even tighter but you're gliding in and out like a dream. You start to pick up the pace, thrusting in earnest, and your thrusts start to fully bottom out. Kelly's ass bounces as your hips slap against her thighs, your swinging balls doing the same thing to her pussy on especially hard thrusts. You can feel the faint sloshing of the lube around you, delighting in the small amount that escapes and dribbles down her legs.

And throughout it all, Kelly talked dirty to you between satisfied gasps, "Oh Fuck yes! Fuck, John, fuck me harder! Fuck my tight ass, OH! Oooh yes! We were made to do this, you were made to fuck me in my asshole! Ungh!" Your brotherly instinct makes you wish you were recording this just so you could play it back to her later for gloating purposes. Maybe next time, depending on how bitchy she was about... well, maybe she deserved to be bitchy about this. Whatever, not your problem right now.

"C'mon sis, you want me to stop? Tell me how you really feel!"

"Oh God... don't... don't you dare stop John! I love it! I love the feeling of you in my ass! I love the feeling of my brothers cock in my ass!" She lets out a high pitched moan and you can tell she's close to orgasm, you're not far off yourself. You're about to keep taunting her when she continues, "It just feels so right! Why does it feel so good? Normal sex doesn't even feel this good!"

"Will you be able to enjoy sex with anyone else again?"

"OOOHhh! N-no... no, I'll never be satisfied with normal sex again John! You ruined sex for me you asshole!"

"Oh I'm so sorry Kelly, how about I just st-"

"Sorry! I'm sorry! I didn't mean it, please... don't ever stop fucking my ass! Make me cum! Again and againandagain-!" That was it, you were at your limit. Seizing your sister by her amazing hips, lift her legs up onto the bed and nearly pull out. In one movement, you haul yourself forward, using your weight and leverage to plunge yourself to new depths inside your sister. "OMIGODYES!" Kelly's blabbering devolves into vigorous grunting as she joins you in orgasm, both of you shaking as you blow your first load of the day inside her ass.

You both lie there, spent but not exhausted, catching your breath before Kelly starts grinding back against your semi-hard cock. You wince, you'd been enjoying the refractory lull, "Kels... can we just take a-"

"No! Keep going! I get that you have to sleep and you can't always be fucking my ass but you have to understand that I'll never feel complete unless you are! Please! Just one more time and then you can sleep, promise!" You get the feeling she's lying. You're pretty sure the only reason she left you alone last night was because you compelled her to. Sooner or later, you'd have to fix this somehow. Plus, there was a hot MILF downstairs you just happened to be related to who needed your attention. You glance down at the ring in your dropped pants. What to do, what to do...