Matt couldn't keep the bargain

Putting sunscreen on your sister. It didn't sound like a big deal. You just smeared a little on and rubbed it in. The only problem was, Karen's bikini left most of her ass and most of her breasts wide open to the sunlight and wide open to Matt's hands.

Doing her back was easy enough. But things, definitely including his cock, got harder when he did her calves and thighs, working his way up toward the danger zone.

He took a deep breath and a new squirt of sunscreen before turning to her ass cheeks, first rubbing the sunscreen between his hands to warm it up before he actually started applying it.

"Thanks," she whispered. "I was sure you'd hit me with freezing cold sunscreen."

A year earlier, he would have. Then again, a year earlier, he hadn't realized what a pervert he was.

Karen's butt was nicely toned, possibly a side-effect of her horse riding. It felt better than he'd imagined it could. Then again, his imagination had only been informed by watching porn videos since this was the first time he'd ever rubbed a girl's naked ass.

He lowered his head a little. He'd read about the scent of a woman and wondered how she would smell.

There was something, a little sweat, a little sweet, a little something he couldn't define. And then he couldn't help himself. He reached into his shorts, grabbed his dick, and tugged.

He came almost instantly, drenching his hand with semen.

Well, what the hell, he thought. He brought out his handful of warm semen and rubbed it into his sister's ass.

"Oh, gross. I can't believe you did that. Especially after you promised." Karen rolled off the lounge chair and punched him in the chest.

Matt couldn't believe it either. But he did notice that Karen's nipples were puckered under her bikini top. She might be grossed out, but at least a part of her was turned on as well.

Interesting.