So there you are, on top of your sister, your johnson plunged all the way inside her. Even if you stop now, even if you take this absolutely no further, this will always be a thing that happened between you. You and your sister will always have a sexual history together.
"You need to stop, big brother," she says, her voice trembling. "DON'T YOU DARE STOP NOW!! You need to pull out of me, this is rape. THIS IS DEFINITELY NOT RAPE, I CONSENT TO ANYTHING YOU WANT!!"
Oh God. You know you need to stop. In a court of law, you know that the mixed signals she's sending will be construed as her revoking consent. But she's so pretty, and the transgression feels so hot, and at least part of her wants it. So of course, there can be no stopping now.
You start to move, tentatively at first, trying to get the feel for your sister's body. As you start to pump inside of her, you place one hand on her breast, and make slow, gentle circles around her nipple. The skin of her tits tightens beneath your touch, and you feel her nipple get hard against the palm of your hand. That feels so good, you start to increase the pace of your movements inside her.
"Mmmm," your sister says, a strange little whimper of pleasure.
"Oh?" you say to her. "What's that? You're starting to like this?"
"I am," she says, and notably, that second voice doesn't speak up to correct her. Plainly both versions of your sister are starting to feel good with you inside her.
Now you're moving inside your sister with the full force of a man who needs his woman. She is smaller than you, and her cunt feels so tight, and you feel the pressure building inside your balls. "This is my first time," you tell her. "I have no kind of stamina at all. If you don't do something to stop me, sis, I'm definitely going to wind up finishing inside you."
Her face twists in sudden, unexpected pleasure at the thought of you putting your seed deep inside her, your own sister. She puts up no kind of resistance; apparently, she's accepted that this will happen with or without her.
"I'm a virgin too," your sister says. "I want you to finish..."