She blackmails him

"You are in so much trouble." Karen grabbed him by the nose. "What do you think dad will say when I tell him my own brother spermed all over me?"

Matt recognized that tone. She didn't really plan on squealing, she wanted something.

"I'm sorry, your body was so hot and I got carried away."

"Typical male. 'Me see something nice. Me grab.' No matter what other people want."

"You're right. I fucked up. So, what do you want?"

"Are you accusing me of ****. That's low."

"You can call it 'hush money' if you want."

She stared at him for a minute... long enough to make him look down to make sure he'd put away his dick after he'd cum on her butt. Fortunately he had.

"Okay, you pay for me to go to the dance this Friday."

"Shit, that's thirty dollars."

"How much would a hooker charge to let you cum on her ass. You're getting off cheap, jerkface."

"Okay, okay. No problem. I'll pay for your dance ticket... under one condition."

"Like you're in any position to negotiate, pervert."

"Nevertheless. I want a dance with you."

"You want to dance with your sister? That's... weird."

"You're the hottest girl I know. Of course I want to dance with you."

"Fine. Now get out of my way. I need to take a shower to get this off of me."

He was pretty sure she was trying to hide a smile when she turned away which was weird but not all bad.