She wanted him to stay

"Well shit, you've already seen everything, pervert. You might as well stay."

"Really?"

"What do you want, an engraved invitation?"

"Uh, no. That's fine. Where do you want me?"

"Take off your shirt and sit on the bed next to me."

He followed instructions, sitting near her waist, so close to her that his jeans brushed against the soft skin of her hip and where he could, without reaching at all, touch either her pussy or her breasts.

Up close, the scent of girl-pussy was stronger. He wouldn't have described it as pleasant but it sure was sexy and he inhaled a deep breath.

"God, you are a pervert."

"I'm not the one who--" He cut off the words before he could say anything stupid. Anything else stupid, really.

"The one who invited her brother to watch her, is that what you were going to say."

"I didn't say it."

"Well shut up. If you want to be helpful..."

"Yes?" Would she ask him to lend a hand?

"Take out your dick and stroke it while I finish."

"I could do that."

"I sort of figured."

She didn't close her eyes. Instead she kept her gaze moving between his naked chest and his newly exposed cock as he stroked it.

She grabbed one of her breasts in one hand and plunged the fingers of her other hand into her pussy, making little moan noises as she drove herself closer and closer to orgasm.

"Give it to me, give it to me, give it to me," she whispered.

For a glorious moment, Matt was convinced she was talking to him but she wasn't. Still, thinking it might be was enough to send him over the edge.

He spurted out cum, then, in a moment of yolo, turned his spraying cock onto his sister.

It might have been coincidence or his semen hitting her chest might have been the trigger she needed. Either way, Karen jerked on the bed, her soft moans turning into a loud groan as she orgasmed.

"That," she said after a minute of panting, "was about the most nasty thing I've ever done."

"More than--"

"Shut up. Now what the hell are you doing up so early and why are you in my room?