He walks in

Matt knew it was a mistake. If he wanted a chance with his sister, he needed to let her think it was her idea. But Matt had spent the past couple of days attempting to control his impulses and once again, he failed.

Karen had locked the bathroom door but the lock was one of those types that could be popped with a screwdriver. It kept him out for two seconds.

He dropped his shorts and boxers on the ground, took his dick in hand, and joined his sister in the shower.

Sure enough, his fantasies, to that point, were borne out. She stood with her eyes closed, one hand deep inside her pussy while the other grabbed both of her tits.

It took him two strokes before he shot his wad over her stomach and the trimmed patch of hair that covered her pussy.

"What the fuck?" Karen slapped him in the face with all of her strength. "Get the hell out of here."

"But... you were masturbating..."

"Do I walk in when you're masturbating? No. Do I ask what perverse fantasies you masturbate over? No. God, Matt. I thought you were better than those creeps at school but you know, you're worse. Now get the shit out of here and leave me alone. If I see you even looking at me funny, I'm telling dad what happened and you can say 'hello military school, goodbye virgin butt.'"

"I--"

"I don't want to hear it. Get the hell out of here."