You can't muster much resistance

"This is fucked up," you said, fighting to keep your shirt on, a process made awkward by the beer can in your hand. The struggle caused Lisa's breasts to jiggle playfully before your face and your sister's body was still grinding away on your lap.

She immediately abandoned the shirt to open your belt, ripping the whole thing free of its loops before you can finish pulling your shirt back down. "Quit teasing me, John. I need this. And I can feel how hard you are," she said with a grin. She popped open the button of your fly.

Your hands blocked hers from pulling the zipper. "Woah, stop it. You're my sister." In your head, you meant that the body is your sisters, but as the words came out you felt your shameful lust for the body blend with your usual irritation for the your sister, the person. Your cock throbs with a confused desire to fuck the girl like you hate her.

She smiled again. "I know, it's so fucking hot, isn't it? Forbidden. Delicious. Look." Shrugging off the suspender straps, she ripped open the front of her little blouse. Your sister's perfect tits came into view, filling your vision. They were incredible. Possibly the best you'd ever seen. You felt your resolve falter and you groan, betraying your rising lust.

Lisa took the beer from your hand and downed the last of it, giving you a fantastic view of her body while she tilted her head back. The can was tossed aside, clattering across the hardwood floor as her focus returned to your face and she slowly dropped the shirt down her arms to the floor, sitting topless in her tiny skirt and stockings on your lap. "Don't be shy. Touch me."

She looked like some kind of bimbo whore, dolled up with the pink lip gloss and pigtails. It helped, actually, to disassociate the person before you from your sister, but you're still reeling from the fact that this was even happening at all. You don't really fight her when she gently places your hand on her breast. Unable to stop yourself, you flex your fingers, groping and squeezing Lisa's flesh. She purred with delight as you join the other hand to the first and openly fondle her chest.

Somewhere in the back of your head, you felt yourself hit a tipping point of no longer caring that the fingers pulling at your zipper belonged to your sister's hands. The body straddling your lap was incredible. Her soft, eager moans of encouragement as you brush your fingers across her stiff nipples fill you with the most shameful arousal of your life.

When her hands finally made it inside the front of your pants, and those fingers curled around your aching cock, and those shimmery pink lips parted to a look of genuine wanton need, all you saw was a sexy slut aching to be filled by your cock. She wanted you, hungered for you like no girl you'd ever been with. It resonated powerfully with something deep and primal.

Caressing your erection the best she can in your position, she tugs on your jeans with her free hand. "Take off your pants. I want your cock. Right now."