He jerks off as he listens

God, he wanted to walk in, to join Karen in that shower, to watch her as she came closer and closer, finally reaching her orgasm as he covered her body with his cum.

But Matt was smart enough to realize that if he was going to have a chance to screw his sister, she would have to think it was his idea. Walking in on her when she clearly needed privacy would give him a short term thrill but would destroy any chance of ever getting the whole thing. Besides, Karen could be a big help for him with the other girls. The last thing he needed was for her to let the other girls at school know that he was a mega-perv.

That said, he couldn't walk away. Yeah he'd just cum, but he was also eighteen. He took his dick out from his shorts and stroked himself as he listened to his sister.

She kept her voice as muffled as she could but the closer she came to orgasm, the less control she had.

He timed his strokes to match her volume, finally letting himself come when he heard her scream.

He'd just wiped himself off with a kitchen towel when he heard his mother's car outside.

Looking down, he saw his boner was obvious but at least he'd cleaned up his semen. And he had to admit he spent a lot of time walking around with a boner. That wouldn't shock his mother.

He opened the door. "Need any help carrying anything in."

His mom kissed him on the cheek. "Thanks, Matt. But I don't have anything."

"Uh, no dinner."

"Oh, crap. I'm so sorry. You did get the free meal at school, though, right."

"Yeah, sure mom."

"Perfect. There's always bread in the freezer and I think there's some peanut butter left."

"I'll make something."

"I know you're a growing boy... uh, man." Oops, yes, she'd seen his hardon.

"I'll be fine, mom."

"Have you and Karen done your homework."

"We got started but... still have some more to go."