It's a secret.

"Oh don't worry, big brother. Little Zoe has the best idea. It's going to be tremendous. But you don't need to worry about it right now. Let me make some preparations first." Her smile makes you nervous. Hell, what about this situation isn't making you nervous!?
Suddenly, she leans down and pops her lips over yours. Before you can even think, she's making out with you: running her fingers through your hair, grinding on your exposed cock with her crotch. You can actually feel the warmth of her cunt through her shorts. Her cunt?! That's your sister, you sick bastard! But no little voice in your head can stop your heart from pounding. You find yourself placing your hands on Zoe's hips, and sliding your fingers up under her t-shirt.
"Ah ah ah." Zoe breaks the kiss. "Don't get any ideas, mister!" She lightly slaps your cheek. "I just thought you looked so tasty for a second. I couldn't resist." She giggles as she climbs off. "Now, do me a favor. No jerking off. Not for the rest of the day. No sex, no porn, no little "fireman" time." She blows you a kiss and turns to go, swaying her big booty in an exaggerated fashion. You are mesmerized by her squeezable ass. How did she manage to even get those shorts on over those huge tracts of land? You taste weed on your lips, and your cock is standing at full attention. You are so confused right now, but you find yourself nodding.
"O....ok." it's all you can say.
After sitting in stunned solitude for another minute, you shake yourself. This day is turning out very strangely, and your in-laws and daughter aren't even home yet. There' a lot to do, you should put your dick away and get to it.

You go about the business of getting ready somewhat mechanically, your head is still in a daze. You come downstairs in time to see Zoe climb into an Uber. Now where is she going? As you begin to tidy up, throwing out the trash and straightening up, Tara O'Day, Jared's busty girlfriend, emerges from his room, smelling of weed and wearing only acid washed cutoffs reminiscent of Zoe's, and a tight black v-neck t-shirt that exposes a large swath of cleavage.
"Excuse me..." she rubs past you to open the fridge and pull out an Arizona Arnold Palmer can. You grumble inwardly: those are yours. They aren't for her.
"Just you and me, huh Mr. S?" she pops the top, drinking long and slow as she paces, barefoot, from the kitchen to the living room. The open floor plan meant that anyone in the kitchen could see the 4K flatscreen in the recessed living room, or the dining room table, or beyond that to the sliding doors that led to the back deck.
"Yeah, I guess so." You watch her warily as you pull the trash liner, and replace it with a new one.
"You don't mind if I put on SportsCenter, do you?" Apparently, Tara was a Dodgers fan and liked to follow all the highlights. If she wasn't such a teasing little tart, it might have been a source of common interest. Though, truth be told, you were an AL man through and through.
"Go ahead, but please make sure it stays clean in there. Trying to get ready-"
"For family. I know, Mr. S. Your such a good, dutiful husband to follow Jenna's instructions." She's leaning over the back of the couch now, infusing every word with a smug little smile.
You've had enough, so you move on to the backyard, checking the pool and the hot tub. {if TaraSFantasy = 1} After jerking off in the shower to the little tart, you don't really want to put up with any flirting.{endif}
Soon after, Zoe returns to the house in her own car, a beat up Malibu, with a few extra bags. Ah, so that's where she'd gone.
"I beat Jenna back!" she announces to no one in particular. Tara doesn't look up from her phone.
"Yes you did. Now help me clean this place up."
When Jenna returns, you all pitch in to help put away the massive load of groceries. As soon as the last item is safely in the fridge, you hear the twin honks of two vehicles pulling into the driveway.