Jack cums first.

It appears that the high intensity fucking is too much for Jack to handle.
"Aw...fffffffuckkkk!!!" He bellows, and Jenna gives a grunt of surprise when she feels Jack's load splatter inside of her. She gives you a look of blended lust and just a bit of a wry grin. You jam your fingers into her mouth as you piston harder and faster into Karen.
"Sorry, sis... orgasm control isn't...really his...strong....fuuuuu....." Karen's explanation is interrupted by a rising scream. "AW FUCK I'M CUMMING.... AHHH!!!!!!" Karen's leg drums on the bedspread as her pussy quivers around your cock, and you feel the satisfaction on driving the redheade sexpot to the heights of ecstasy while her husband staggers away towards the bathroom, spent. After screaming out her pleasure for a few more seconds, Karen collapses onto her belly, and you pull out, giving her ample asscheeks a squeeze.
"Good job, baby..." Jenna mumbles, laying on her side. You look at her sweat-slicked body, and immediately climb on top of her. Your steel hard erection hasn't been satisfied yet.
"Ooh, baby..." she moans as you slide into that familiar tunnel, still slick from the juices of her sister. "Give it to me..."
You give it to her, alright, fucking her in the special ways that only you know how. After a few minutes, you feel Karen's hands on your chest from behind, rubbing your body and kissing your neck from behind. "Fuck her good, Jameson..." she slides around to pinch Jenna's nipples and make out with her while you fuck. You switch to fucking Jenna doggystyle, and Karen takes advantage of Jenna's available mouth to recieve some oral from her younger sister, all the while making the most obscene faces at you. You can't hold up for too long in that circumstance, and find yourself groaning as you blast a load of cum into Jenna, while she screams and shakes in an orgasm of her her own, with Karen squealing in pleasure, adding to the chorus of pleasurable noises.
When the pink mist clears, you find yourself lounging on the pillows, panting, with a sister on each shoulder. As you wallow in the adolescent heterosexual fantasy, trying not to be wierded out by the "sister" aspect of the situation, you chuckle suddenly.
"What's funny, baby?" Jenna asks languidly, and you point to the armchair where Jack, silly Jack, is passed out, naked and soft. Karen gives a snicker."
"I kind of expected that. Sorry, Jen." She sighs, circling your nipple with a finger idly. "Between the drinks and everything, I was pretty sure he wasn't going to last beyond his first nut."
"It's okay....." Jenna says, her hand traveling southward until it wraps around your semi-soft cock. "I think there's plenty of live cock to go around."
Karen's smile is wicked as she slides from your shoulder, shimmying down the bed while Jenna begins to stroke.