Below is the original version of the fiction. Click Next for a completely rewritten version of the same idea. I recommend skipping this version here and reading the newer version. Or skip this fic altogether if you think the concept is dumb. You do you.
Day 1:
One day, Grunkle Stan arrives at the Mystery Shack with a new hot tub being carried by fork lift.
Dipper and Mabel notice, and take a look.
Stan: That's it, just a little more, stop, right there.
*hot tub is put in place*
Dipper: Wow, a new hot tub?
Grunkle Stan: Don't get any ideas kids, dis hot tub is for myself only.
Dipper and Mabel simultaneously: awwww...
Stan: Now get back to work kids!
*Stan walks off*
Mabel: Gee dipper, I've always wanted a hot tub.
Dipper: Same Mabel, shame that Grunkle Stan is hogging it for himself.
Mabel: Yeah, what a shame.
Later that night.
Both twins asleep in bed.
Mabel awakens from her fake sleep and gets up and changes from her floppy disc night gown into her swim costume.
She walks outside in the dark of night, turns on the hot tub, and gets in.
Mabel: ahh... paradise.
She enjoys it to say the least.
She gets out later that night, and goes to back to bed.
Day 2.
The next day, Mable seems happier than usual.
Dipper: Gee Mabel, you sure seem more upbeat than usual, and that's saying something
Mable: What can i say? I'm just a girl with a can do attitude.
Dipper: Whatever. Say, you didn't... sneak into Stan's hot tub did you?
Mabel: What... no. Of course not. Why would I? *nervous*
Dipper: uh huh...
Mabel: Oh hey look, it's Candy and Grenda, see ya tomorrow Dip, goin' to a sleeping over.
Dippy: hmm... (Dipper gets an idea).
Night 2:
Dipper notices he is alone in his bedroom, so he does the only logical thing he can think of.
Dipper plans to sneak into the hot tub, but seeing as he's alone, he also wants to make the most of it. So from his normal sleep attire of a shirt and shorts, completely strips down, wraps himself in a towel, and walks down to the floor, walks outside, and then he gets into the tub and skinny dips in Stan's hot tub
Dipper: aaaaaaaaahhhhhh...
He enjoys it. Maybe a bit too much...
Later that night, he sees it's late, he gets out, puts his towel back on, and walks back upstairs.
Day 3:
Dipper is in an unbelievably good mood.
Mabel: I'm baaaaaack!
Dipper: *just sweeping floor with huge grin*
Mabel: YOU are in a good mood today
Dipper: Oh. Am I really? I hadn't noticed.
Mabel: You're strange.
Dipper: Say what you want Mabel, but let me tell you, last night was magical.
Mabel: Yes... do tell.
Dipper: Not literally, just figuratively... *voice trails off as Dipper feels he's already said too much*
Mabel: mmm hmm *gives Dipper a suspicious look*
Night 3:
Unlike previous nights, BOTH twins had now used the hot tub now, but at different times. Mabel missed a night, but the night she missed out, Dipper had skinny dipped in the tub now that both twins experienced true bliss, now that both of them had the same idea...
Just like the first night, Mabel changes from her night gown to her swim costume (inside her bed room with dipper in the room),
but this time, instead of just walking down, to be on the safe side, she rearranges her pillows under her blanket to make it look like she's still in bed. she then walks down stairs quietly, and into the warm and steamy hot tub.
Slightly later, thinking Dipper to himself that he's so clever by using the tub especially late at night, does what he did the other night. Fooled by Mabel's decoy, and thinking she is still asleep, he gets out of bed, removes all his clothing, dawns a towel, and walks down stairs.
By this point, Mabel is already in the hot tub.
Dipper to Dipper walking towards tub: ah, cannot wait to get into that tub and experience paradise.
Dipper arrives to tub and past the steam, it is then that he experiences true horror and shock.
Dipper: MABEL?!
Mabel: Oh hey Dipper, here to use Stan's property without permission too?
Dipper: Uh... yeah... uh huh...
Mabel: What's wrong Dipper? You seem nervous, and your checks are turning red.
Dipper: Its... just... *continues to blush*
Mabel: Just what?
Dipper: It's just... had I known you were going to be here...
Dipper: I would've have dressed better... or at all...
Mabel: Wait, don't tell me.
Dipper: ...yep... *in tents blushing*
Mabel:... You're naked!
Dipper: ... (too embarrassed to say a word)
Mabel: *giggles*, that's cute.
Dipper: ... (still too embarrassed to speak)
Mabel: Well what are you waiting for?
Dipper: ...huh?
Mabel: You don't have to leave on my account.
Dipper: Really?
Mabel: Sure. Tell you what, I won't look while you get in *covers eyes with hands*.
Dipper: No no, let me just go inside, it'll only take a couple of minutes.
Mabel: Just get in all ready!
Dipper: Okay! Okay.
Dipper: Pauses for a bit, and takes a deep breathe. he removes his towel, and enters the tub naked as a new born baby.
Dipper *giggles a bit in embarrassment*
Mabel: I don't get why you're so shy, I'm your sister, I've seen your junk before. Heck we've even bathed together before.
Dipper: Yeah, when we were like... 3.
Mabel: 5.
Dipper: 2.
Mabel: 6.
Dipper: 1.
Mabel: 7! we bathed together when we were 7 remember?!
Dipper: You have no proof!
Mabel: Do you want proof? Cuz I have proof. *smirks*
Dipper: Whatever. Either way, it's been a while.
Mabel: 6 years is a "a while" lol.
Dipper: Think about it, 6 years out of our soon to be 13 years of living, that's like a big percent of our lives.
Mabel: Well, either way, you're just so... on edge. Relax for bit, unwind.
Dipper: Yeah, unwind...
(the two sit and chilax for a bit in the tub. dipper was a bit embarrassed being naked in front of his sister,
but she couldn't see below the water, so he got adjusted to an extent)
some minutes later...
Mabel: You know, I'm kind of jealous you thought of skinny dipping, I would have done it too had i thought to do so.
Dipper: heh heh heh heh heh (nervously).
Mabel: You know...
Dipper: Do i want to know?
Mabel: Since it's just you and I here...
Dipper: *gulps*
Mabel: *blushes a bit then says* You wouldn't... mind if i stripped would you?
Dipper: *blushes a bunch* Uh...
Mabel: Never mind, dumb question... (she says putting herself down a bit)
Dipper: No no no. Um... sure.
Mabel: *pauses*
Dipper: I mean... sure I don't mind. Like you said, we're siblings, and after all, I've seen your junk too.
Mabel: *giggles* heh, I guess you're right.
Dipper: I'll close my eyes while you change *covers eyes*
Mabel: *smiles*
Dipper: *hears unzipping sound*
Mabel: ...you can look now.
Dipper: *opens eyes* *sees Mabel's swim suit sitting on the side of the tub*
Mabel: Well?
Dipper: Well what?
Mabel: How do you feel?
Dipper: How do i feel?
Mabel: Yeah.
Dipper: I don't know. Sweaty? Nervous? Something along those lines.
Mabel: Look, quit being so tense. The whole point of dipping skinny was to relax, and so that's what I want us to do.
Dipper: Yeah... *still blushing a bit*
Mabel: I know it's been a while since we've... you know... done this. But to be honest.. I do miss those days.
Dipper: ...really.
Mabel: Yeah, back in the good old days of early childhood, back when there wasn't a care in the world,
and going around naked was somewhat more acceptable.
Dipper: heh heh, yeah, those were the days. *scratches head*
Mabel: You haven't said a whole lot other than just respond to me. Anything you want to get off your chest?
Dipper: Well... If you must ask. I have to say... Mabel...
Mabel: Yeah?
Dipper: About our age... the thing is...
Is our childhood even over? I mean we're still technically children, the more you dwell on the past, then less time you have to enjoy the present. And who knows if life'll get any better from here on out.
Mabel: *silent*
Dipper: ...just think after this summer is over... we'll be teenagers.
Mabel: That's pretty scary to think about.
Dipper: It is. Also...
Mabel: Yeah?
Dipper: On a more positive note, this has been one of the best summers ever, and it isn't just because of the mystery or the trill, it's because of you.
Mabel: Really?
Dipper: Yeah. Really.
*the two stare at each other for a bit eye to eye*
*their hands meet underwater and they hold hands*
*they both close eyes, and lean back in pleasure, and smile*
*they get soo comfortable, that they both fall asleep in the tub*
Day 4
Grunkle Stan: oh KIIIIIIIIDS!
Dipper and Mabel simultaneously: huh? *scream*
Stan: What's the big idea? Using MY hot tub that I specifically said was MINE alone.
Dipper: Come to think about it, have you even used this thing yet?
Stan: I was planning to today, but instead I come out and see this!
Dipper: We... we're sorry Grunkle Stan.
Mabel: We just thought it was a shame you had to hog it for yourself.
Stan: *sigh* Well, whatever, just get up, get out, get dressed, and get back to work.
Dipper: but it's Sunday, shack is closed.
Stan: I said work!
*Stan starts to walk away*
Dipper: Wait so... that's it?
Stan: Well normally I'd punish you delinquents, but I figured the embarrassment is punishment enough. Good luck getting out of this one.
*Stan walks back to the shack*
Mabel: Well now what do we do?
Dipper: Get back inside and get dressed I guess, now where did I leave my t... oh no... Where's my towel? *says panicking*
Mabel: And where's my swimsuit?
*both scream*
Mabel: Did Stan our things?
Dipper: Where else could they have gone?
Mabel: I don't know? Maybe they were blown away by the wind?
Dipper: An unlikely story.
Mabel: Well anyway, now what?
Dipper: Wait until night when everyone's asleep and sneak back in?
Mabel: We could, but I don't think my skin can handle any more pruning. Let's just go inside now.
Dipper: No no no, I ain't getting out of this tub until i am 100% clothed.
Mabel: Well you got quite a way to go. Besides, it's Sunday, shack isn't open to customers today.
Dipper: True, still, what if we run into Wendy?
Mabel: Hello? Sunday; she's off work.
Dipper: She could still show up to hang out or something.
Mabel: Get over yourself, come on *she gets out of the tub fully exposed*
Dipper: *shields eyes* I don't know how you can be so daring.
Mabel: The trick is to not think about it.
Mabel: So how about this, since you're too chicken to get out, I'll get dressed and bring you a towel.
Dipper: Sure that sounds great.
Mabel: It's the least I can do for my twin brother.
Dipper: *smiles*
a few minutes later, Mabel returns fully clothed in her apple bloom sweatshirt
Mabel: Here you are Skinny Dipper *giggles*
Dipper: Please don't call me that ever again.
*Dipper puts on towel, walks up the stairs, and gets dressed*
Dipper: Well, that was a night for sure.
Mabel: mmm hmm, and how was last night?
Dipper: It was, not going to lie, pretty enjoyable, better than the previous night. I feel like i've gotten to know you for the first time again.
Mabel: *giggles*, you're weird.
Dipper: so are you.
Mabel: And I wouldn't have you any other way.
Dipper: Same to you.
*the two give an awkward sibling hug, and they move on with their normal day.
Later in the afternoon, Stan walks up to the twins and speaks.
Stan: Hey um, so kids. I was thinking. You really did seem to enjoy that hot tub.
Dipper and Mabel simultaneously: Yep. And?
Stan: You're right, I may have been a bit selfish keeping it to myself. So how about this. Clean up the shack, and help restock items, and tonight we'll all use the hot tub together. Just so long as you, you know, use it like civilized human beings.
Dipper: Sounds great.
Mabel: Thanks Grunkle Stan.
Stan: Heh heh, just don't expect any hospitality from me too often. Now get back to work.
The twins finish all their chores, watch some TV, and indeed that night use the hot tub with their Great Uncle Stan. Although after seeing Stan's old man body, they have second guesses, but eventually enjoy themselves as one big family. And yes all three of them had the decently to dress properly.
And thus concludes our tale of something some.