She's persistent

"Daww. Is somebody still mad for me beating them at Twister?" she says in a mocking, whiny voice.
She puts her hands on your butt and starts petting it like a dog to make fun of you, but then she gets curious again.

"You know..." she says as she gives your butt a small squeeze "..my sister did get to see you naked...."

"Uh-huh." you say cautiously, unsure of where she is going with this.

"I think it's only fair, I see you also." she says very matter-of-factly.

Completely tired and so over dealing with this girl, you think what the hell? You're mature enough. "Fine. If you promise you'll just go to sleep afterwards."

"Deal!"