Find Karen instead.

A sudden, mischievous though occurs to you. You pop your head into the guest room. Neither Karen nor Jack are there. You look out the window: Jack's car is not in the driveway.
You trot downstairs. Sounds of splashing pull you towards the sliding doors, and you can just see Allison and Riley in the pool, getting an early start on some fun. The pool's probably warmer than the air right now.
The downstairs shower is running. You try the door handle, and smile when you find it unlocked. The clothes stacked on the toilet seat give a pretty clear indication of who's in the shower. You close the door behind you and lock it, and slip off your boxers and tshirt.
"Mmm.... Jack? Is that you honey?" Karen's musical voice rings out over the sound of the water.
"Mmmhmm.." You slip into the shower behind her. Her hair and face are covered in shampoo, and her eyes are closed. You decide to take advantage, grabbing her tits from behind and pulling her tight to your body.
"Oooh... good morning, frisky." She reaches back to grab your cock... and then she gasps.
"J.....James.... is that you, you dirty boy?"
"You caught me..." Karen rinses the shampoo out of her face and turns to look at you. "Wow, Jack must be tiny if you can tell it's me just from one grab of my cock, Kare."
"He's got some, distinctive geography on his..." She strokes yours. "And you are bigger... thicker too." She drops to her knees, kissing and spitting on your cock. "And you shave. Jack thinks manscaping is for gay guys..." her tone conveys her thoughts on the matter. She starts polishing your knob, and you rub the back of her head as she does.
"I needed more of you after last night." You sigh. Karen hums against your cock, and you shudder.
"Tell me the truth, James." She looks up at you as she strokes your cock. "Who gives better head? Me or Jenna."
"I don't know, Karen. I don't think I have enough data to make a proper comparison..." you smile down at her.
"Oh, well then..." she strokes a little faster, "Lets fix that."
She spends the next few minutes performing expert fellatio. "Jesus Christ, Karen..." she's lapping her tongue as she goes up and down, stopping to suck your balls and really stimulating the right spots.
"I don't get a lot of practice..." she stops to gasp out at one point. "Jack doesn't keep it neat and clean, so I don't blow him that much these days..." she resumes.
"You're easily as good as Jenna, Karen. With practice... you could be better." It's true. Jenna was exceptional, but this... if this was Karen rusty....
"I'm going to cum."
"Not yet you aren't." She bends over in front of you. "Not until you fuck me, stud."
You slide inside without a second thought, pounding your sister-in-law's pussy.
"Fuck..fuck...fuck me.... James..." she reaches back to grip your hip with one hand, bracing against the wall of the shower with the other.

"I wish I could have you all to myself..." she moans as you pound her deep.
"Greedy..." you pant.
"I know... I shouldn't tell... mm mm mm m... you this..." she looks back. "But I've always been jealous of Jenna. She got the good one..."
"Oh fuck...." you grunt and groan as your cock twitches. Before you can even ask... you explode deep in Karen's pussy, and she wiggles and twists on your cock.
"Give me that spunk, Jameson..."
You sit down hard on the tub's edge, watching as your cum leaks out of Karen's hot pussy.
"Mmm... I love it." She gives you a kiss. "Now get out of here before someone catches you."
You towel off, redress and sneak back into the kitchen to pour yourself some cereal.

While you're eating, the girls come in from outside, and Jenna trots down the stairs as well. You are a little embarrassed to be in your boxers in front of Allison and Riley, but neither seems to even notice.
As soon as breakfast is done, Karen and Jenna head out to meet the realtor. Tara comes out from Jared's room, and you tell her to sit tight, and that you'll call a tow truck to come and take her to her car.
"I don't...I don't know how I'm going to afford it, if it's a big thing..." Tara looks upset.
"Don't even worry about it. We'll work it out, whatever it is."
Tara gives you a big smile, and her eyes get a little glassy.
Once Tara leaves with the tow guy, you and Jared head out, in separate cars, to the country club.