On his sister? Gross

Matt smeared some sunscreen on Karen's back and then moved down to her calves and thigh. He drew the line, though, when it came to her naked butt. This was his sister, after all.

"Sorry," he said. "You're going to have to take care of those cheeks."

"What? I thought you would be drooling all over my ass."

It was a nice ass. Obviously her hours spent riding had toned her and, just for an instant, Matt had a fantasy about how those tight ass-muscles might feel squeezing down on his dick. But eeeew. Karen was his sister and he certainly wasn't going to butt-fuck her.

"Look, you've got a great body, all right. Now, let's see if we can get the trig thing down."

Karen took the sunscreen from him, squirted a couple of globs out... damn, that looked like a couple of globs of semen... and liberally applied it to her butt. Then she licked her lips... and grabbed her trig book. {if amy-tutor=1}"Give it to me the way you gave it to Amy, all right?

{else}I think I'm getting this but Ms. Chadra is sort of distracting, isn't she.{endif}

Matt went through the homework with his sister. He was sort of proud of himself for resisting the temptation to feel her up, and sort of ashamed that a part of him even felt like this was something to be proud of at all. I mean, how many guys found their own sisters attractive?