Give Kelly what she wants.

Are you really going to pass up your sister's offer of anal? Of course not! Besides, who could say no to a begging woman?

"Fine, I guess," you say, watching her. She blinks and stares up at you. You stand up over her.

"Really?" she says incredulously.

"Yes. I mean, I guess. If you really want it."

She smiles at you. "Thanks, bro." She wastes no time and moves over to you, starting to unbuckle your belt. "By the way. There's nothing sexual about this. I mean, it's not like I'm attracted to my own brother. If I had a choice, I wouldn't be doing this." She finally undoes your pants and slides them down. Your cock springs up, already at full mast from seeing your sister beg for sex. She brings her mouth to start licking at your dick. "I really don't have a choice... *slurp* I just really need... *slurp* your cock in my ass. Nothing sexual about it. It's almost like... *slurp* Like you're doing me... *slurp* a favor. It just happens to involve... *slurp* fucking my ass."

You chuckle as you watch her licking your cock. "Which one of us are you trying to convince?"

She frowns and gives you a nice, long, slow lick up the underside of your cock. She sighs. "I don't fucking know anymore. All I know is that I fucking need it."

She gets up and lays on the bed. She sticks her legs far in the air and reaches down to spread her cheeks. "Stick it in me. Stick your dick in my ass. Right fucking now. My ass feels so empty without your dick inside of it."

She doesn't have to tell you twice. You go over to her and, gazing down at her naked body, glide your cock right to her puckered rosebud. You tease her hole with your head for a moment before popping it in. She starts groaning a little as you thrust deeper inside of her.

"Yes, that's it," she says, encouraging you. "Stick it all the way in. Stuff my ass full of your cock."

It takes some pushing but, finally, you're able to thrust your cock all the way to the base.

Your sister is panting from effort and pleasure. She wraps her legs around you, pushing your dick even further in. "Okay. Okay. Whenever you're ready, start pounding that ass."

You pause for a moment and take in the sensation. Your sister's ass is so fucking tight and warm. You look at her and she smiles, urging you on with her eyes. Finally, you start to thrust. She sighs, contented.

"That's it," she says. "I feel like my life is complete now that you're finally fucking my ass. It feels so... right, you know? Like my life's purpose was to have you fucking my ass."

You gaze at your sister's body as you fuck her tight butthole. Her tits jiggle with every thrust. You're practically hypnotized by this. Kelly notices and she frowns at you.

"Hey. Y-You're not *gasp* getting some sick pleasure *moan* out of this, are you? This isn't supposed to be -oh FUCK- sexual. It's just a brother -oh fuck, yeah, keep doing that- helping his s-sister out."

"No, no..." You start to say, then moan as you fuck her faster. "Okay, yes. Fuck, Kelly, your ass is so damn sweet."

She moans and nods. "Fucking... fine. Yes, it's like my ass was made for your cock. Glad we're on the same page. Just keep fucking me. Don't stop. Never fucking stop." She bites her lip and groans in pleasure.

You glance down and see your sister start to play with her pussy while you fuck her. She rubs at her clit and gasps with each flick of her finger. You fuck her faster and faster. Finally, her ass starts squeezing you even more as she starts cumming. You find you can't take it and start to cum with her, filling her ass with your seed.

Finally, you start to come down from your orgasm high. You look down at your sister, who has brought both hands up to play with her tits. You shiver and start to pull away. However, she wraps her legs more tightly around you.

"What are you doing?" she asks.

"I-I'm done. So are you. I was gonna---"

"No, you can't. You need to keep fucking my ass. I don't want you to stop. You have to fuck my ass."

You look confused for a moment. Then you realize that your command of having her fuck you with her ass never had a specific reason to stop. Looks like your sis is in this for the real long haul. You have to think your way out of it.

"What if we have to go to the bathroom? Or eat? What if mom catches us?"

"I don't care about any of that right now. All I care about is you fucking my ass. Besides, think of it this way. With your cum, I'll be super lubed up."

You groan at the thought and start to slowly thrust again. Your dick is raw. You've cum a few times today already. You really need a break. But Kelly won't let you. Think. What can you do?