A visitor.

You lean back in your chair after the door closes on Jenna, contemplating a lot of things. Maybe now you should call Kayla back, or get to work on the yard, or clean the grill and take out the trash. You close your eyes in frustration about what just happened.
You hear the office door open again. Jenna has returned, you guess.
"Forget something, baby....?" you trail off as Zoe slips into the room and shuts the door behind her. She's wearing acid washed cutoffs, thigh high black fishnets and black low top Chucks, with suspenders over a Distillers t-shirt--a size too small and showing off deep cleavage--, and clearly no bra as her nipples make two tents in the tight fabric.
"Zoe, holy shit! You gotta knock!" Your hands fly to cover you shrinking dick, which, strangely, is not shrinking any longer, actually.
Zoe moves like a cat, plopping herself sideways onto your lap in a jiffy.
"It's not like I've never seen a dick before, Jimmy." you groan. No one calls you Jimmy, as you hate it... no one except for Zoe. "It's not like I've never seen yours before, either." She had been curious when she was younger, and there had been several... incidents over the years of.... "accidental" viewing.
Your cock is being pressed between your legs and her round butt, and she squirms playfully. "Oooh I can feel that big guy down there." She giggles, but then she turns more serious as she puts her arms around your neck and looks into your face. "I was watching you guys. Jenna's so fucking sexy. But more importantly...what gives? You having...trouble?"
You can't actually believe this conversation is happening. Then again, frank discussions about sex have never been something Zoe had any sense of propriety about.
"I'm....i'm not discussing that with my little sister, Zoe. Can you please get off, you're hurting...me." She shifts her ass so that can pull your dick to rest upright, but doesn't get off you.
"James, you can tell me about it. What's the matter? You need Cialis? I can hook you up on the cheap, man, you don't even need to go to a doctor."
Zoe isn't going to let this go. You can't believe this, but you start to explain. "I can get it up, Zoe! It's just...sometimes I have trouble getting to the end from just sex."
"Oh....jerkin' it too much, huh? I perused your computer downstairs last night. That's a lot of porn for one guy." Your face couldn't be turning more red. How had she guessed your password?
"Whatever it is, it's my problem. My problem and Jenna's." She gives you a look of true concern amid her wicked smiles.
"Well...." she swings one leg over, so that now she's straddling you, and her tits are directly pressed against you as she leans closer. You are uncomfortably aware that you cock is pressed directly against the crotch of her jean shorts. She even grinds a little, and you squeeze your eyes shut. "Maybe we can come up with a solution."