Leave her to it.

Well, she doesn't need to micromanaged. You imagine it will be better and hotter, to wring all the dirty details out of her later while you pound her cheating pussy. This new dynamic is turning out to be more fun that you anticipated. You snap a quick photo of the pair for your own enjoyment later.
You tiptoe away, enjoying the sound of Jack's muffled moaning. You spare a thought for poor Karen, down at the pool while her husband is getting a world class blowjob from her sister. Perhaps...this situation could be turned into a little fantasy fulfillment for you? You'll have to think on it.
You return to the pool with a boner tucked into your waistband and a skip in your step.
"Well? Where's Jenna?" Karen asks as you trot down the stairs. Zoe has made an appearance now, swimming and splashing with Riley in a white tshirt and sport panties. You guess she didn't grab a bathing suit.
You park yourself in the chair next to Karen. "Beats me. Couldn't find her." You turn to look at her. "Decided to come back and hang out with you instead. It's a lot more fun anyway." You ply her with questions about work and life, and the possibilities of moving to a new place. Their lease is up back home in August, so they have until then to find a place here.
"We can help out with anything you need, Karen. Advice, a place to crash, anything..." you take a chance and place a hand on her knee. She doesn't seem to mind; she even smiles.
"Sometimes..." her voice is quiet... "no, I shouldn't." Her face says that she wants to say, though.
"What? You've started already, out with it."
Karen looks around. Allison is back in the pool with Riley and Zoe. None are paying you any attention.
"Jack... he's a... good provider, and a good father to Allison..." her voice is quiet. "And we have a very nice life... but I don't know. He's such a... typical man. He's insensitive, materialistic, and can't keeps his eyes, and sometimes his hands, to himself. I don't know if you saw him earlier with that little tart that your son is dating..." {if HelpKaren = 1
}
"I did..."
{else
} You shake your head. "Well he was nearly drooling right into her tits." she rolls her eyes. {endif
}"I know that I'm not some wrinkled up old crone."
"Far from it, Karen." you risk a little squeeze of her knee, and a small slide upward to her thigh. No protest from her.
"But when he does that shit..."
"It doesn't feel great, I get it." You grab her hand with your other hand, and give it a squeeze. "Jack's just a stupid guy, Karen. He'd be crazy not to appreciate what he's got. Sometimes even the best of us can let our dicks take the lead." You think about what's going on upstairs. Jack was certainly letting his dick lead right now...probably right into Jenna's cunt.
"Well, you seem like you know how to make a girl feel appreciated much better than Jack does, I won't lie. Jenna's got a good thing with you." she places a hand on the back of yours, the one that's firm on her thigh. "I'm... kind of jealous."
The moment is broken by a big splash as Zoe picks up Riley and suplexes her into the deep end. You pull your hand back, and Karen adjusts her position in the seat. Her face is blooming a little crimson.
"Sorry...I shouldn't..."
"It's okay, Karen. Honesty is never a bad thing." You see out of the corner of your eye, Jenna emerging from the sliding glass door. Not a hair is out of place, with no sign of her little fling. She joins the group amid adulation from Riley and Zoe about her body and the sexy bikini. Karen watches Jenna with....envy?
"It's not fair..." she mutters under her breath. "That body at her age...and a ho... good..husband." she chokes on the word as she glances at you. You put a hand on her knee once more.
"You've got a killer body, too." You risk a quick slide up the inner part of her thigh, releasing before Jenna gets to you.
"Having a good time down here, slacker?" Jenna twits you as you rise to give her a hug and a kiss. As you pull her close, you whisper... "I know what you were up to, bad girl."
When you pull back, her face is pure animal lust for a moment. She smiles, and tries to effect an innocent face.
"I'll deal with you later." you put a finger on her lips. Louder, you say "I'm getting a drink, why don't you take my spot?"