Incest Seduction/Take Kim up on her offer

"Wait a second. You just grabbed at my dick and you thi-"

"By accident..."

"Yeah, 'accidentally'..." you scoff, still petting her. "And you expect me to just ignore that by offering up reciprocation?"

Your cock is still growing and Kim has made no effort to move her head from your lap. You can already feel yourself pressing up against her face. She slowly nuzzles her head against your crotch, her cheek rubbing you through your pants. She's still facing away from you but rotates herself enough to open up more of her body. She doesn't say anything, clearly expecting you to take her up on her offer.

Your sister knows you too well.

Thinking purely with your cock, you slide the hand you have at her side onto her stomach and push upward under her shirt. You can feel her body tense up beneath your fingers as you bump up against her lacy bra. Considering how brazen and shameless she was with you just now, you should respond in kind.

Grope your sister's little tits through her bra.

One-up your sister; slide your hand inside her bra.

Stopping at Kim's bra you realize this isn't a fair trade; you should be going 'clit'-for-tat, not tit-for-tat. Head south instead.