Cassie does

A moment of hesitation, then Cassie pushes Adam onto his back and jumps on top of him. She lowers herself onto his hard cock, then proceeds to ride him hard and fast, drawing an involuntary moan from him as he realizes how good it feels inside her.

Wow, Adam thinks, she really wants to win. He tries to pull himself together, but all he can think about are Cassie's luscious breasts jiggling in front of him as she rides him cowgirl style. He realizes that at this rate, his sister's going to be able to ride him to orgasm before long. He needs to strike back.

"Come on," she moans. "Come in my pussy. Come in your sisters' pussy." Cassie says. "You know you want to."

Sister, Adam realizes. He's fucking his sister. He can't come inside his sister, of all people. He'd never live it down. With a new determination, Adam regains his senses. He starts thrusting back into her, making her moan even harder.

"You like that, whore? That's right, whore, fucking your own brother for money? Come on my cock, you dirty slut."

"Fuck fuck fuck" she says, losing control. Adam can see that she's distracted. He reaches up and grabs one of her tits, making her moan in ecstasy as she seems on the verge of orgasm.

Adam's stamina is rapidly fading too. His thrusts are getting weaker and weaker, and despite Cassie being his sister that just makes her even sexier. It isn't long before Adam feels himself on the edge, the two of them stuck in a **** battle at peak arousal.

"Oh my god oh my god" she moans.

"Fuck fuck fuck" he groans.

He needs to make his move quickly.