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Dean knows Sam inside and out, so when he notices all of the little tell tale signs such as his legs slightly bouncing and him tapping his fingers against the window in a weak attempt at distraction he can immediately tell what the problem is.
Sam needs to pee, which is just great because they are currently driving in the middle of nowhere with the next rest stop being miles away.
Luckily enough, this time they're prepared.
Since Dean was honestly sick and tired of having to pull over and make stops because Sam had a surprisingly small bladder for a guy that freakishly tall he decided to put him in a diaper for trips this long. So that they didn't have to stop.
The is wasn't entirely the reason Dean wanted Sam in diapers though.
Dean loved his baby brother, and even as he got older and older Dean always saw him as someone to take care of and let's just say that at some points he got his wires crossed and that parental responsibility somehow began taking a sexual turn… it started when Sam accidentally called him Daddy during sex and only went on from there. He managed to convince Sam to let him baby him, to suck on a pacifier, to let him bottlefeed him. To punish him as if he were a child when he "misbehaved" and so on.
Of course, all of that took quite a bite of convincing but Dean was very patient. Which led them to this point.
The whole diaper thing led to a small fight, Sammy was quite against it because there was only so much babying he could take but Dean was able to convince him with a few solid arguments and maybe a bit of guilt-tripping "I've sacrificed my life for you over and over and you won't even do this small thing for me?"
So, this all led to Sam now wearing a diaper. If Dean looks closely enough he can see the white plastic waistband peeking over the top of Sam's jeans. It doesn't bulge under his jeans though, so overall Sam should be grateful Dean picked something that's not too obvious.
Even though Dean convinced him to wear it, he still seems reluctant at the idea of using it.
But Sam must know that they're not anywhere close to a rest stop because he hasn't asked Dean to stop at the next one yet, and he's not the type to ask to pull over so that he can go in the woods or something. Unless he's truly desperate. So it must'nt be that bad yet.
That's fine, it'll take a long time for them to get where they're going. Dean is sure he'll reach that point during that time.
And he's proved right, as they drive further down the road with no real civilization in sight he watches out of the corner of his eye as Sam gets more and more antsy. At some point he crosses his legs for a few minutes before uncrossing them after it's stopped helping, and going back to squirming.
"Something bothering ya?" Dean questions as Sam 'discreetly' repositions himself for the umpteenth time.
"What? No I'm fine." Sam tells him which does nothing to convince Dean.
"You sure, I mean you're looking a little… antsy over here."
It's honestly hilarious to watch as Sam suddenly stills as if that could make Dean forget all the earlier squirming.
"Yeah well, just tired of sitting in the car that's all. You know how it is." Sam says unconvincingly, he must know Dean can see through his bullshit right?
"Right."
Dean doesn't push the issue further, but now that he's verbally acknowledged that Sam had been acting off he gets to enjoy his brothers weak attempt at acting like he doesn't need to use the bathroom. He's still, too still for it to be natural. Clearly overcompensating for earlier behaviour. The clear discomfort in his expression would've concerned Dean if he didn't know the reason for it and since he does he only finds it amusing.
There's a clear solution to his issue, if Sam won't give in now that's his problem. He'll have to give in sooner or later anyway because Dean is not pulling over anytime soon.
Sam's lack of movement only lasts for a whopping fifteen minutes and then he gradually starts squirming again. It's even more obvious than before. How much did he even drink? They shared a beer at the motel before leaving, and then they went to get breakfast and Sam had a tall glass of water with his salad. And then Dean got them both cans of soda from the nearest vending machine before they got back to the road. Sam's empty can rests on the floor.
Other people might find it odd how much he keeps track of his brothers drinking habits, but he's just being an attentive caring brother.
He didn't slip Sam extra drinks so that he'd end up desperate quicker, he'd never. He was just looking out, yeah. Definitely.
The miles drone on and Sam seems to be doing worse and worse everytime Dean glances over, he's looking out of the window probably in search of their next rest stop but all that's surrounding them is trees.
It won't take much longer for him to ask Dean to stop, it seems he's reaching his limit.
"Dean?" Sam calls out, as expected. "Could you pull over?"
"And why's that Sammy?" Dean pretends to question, not sparing his brother a glance.
"You know why." Sam says, already seeming frustrated. But when Dean looks over at him with his best clueless expression he finally admits his problem with a sigh. "I need to take a leak, alright?"
"Well maybe you should've gone before we left then." Dean simply says.
"I did go then, that was hours ago Dean!" He seems to get more annoyed by the second, it truly must be bad if he's losing patience this quick. "Come on, I'll be quick."
"No Sam, we can't keep wasting time just because you need me stop every five seconds because you have a bladder the size of a walnut. I'd rather reach the next town before night falls." Dean says, and adds with a smirks "But hey, if it's truly that bad you know you are wearing protection. Feel free to use it man."
Sam's face goes red at once before he looks away from his brother in shame. "I'm not using that! It's bad enough that you convinced me to wear it…"
"Suit yourself, but I'm not stopping. End of discussion."
A sigh follows but other than that Sam doesn't try to argue further, it wouldn't bring him anything either way.
More time passes and Sam gets more and more desperate, Dean is kind enough to not poke fun at him when he accidentally lets out a small whine during a particularly desperate throb of his bladder. If it were any other situation maybe Dean would feel bad but Sam is doing this to himself.
"Dean please." Sam pleads, breaking the silence. He's looking at Dean with those puppy dog eyes and Dean has to do his best to look away. "Please pull over, It'll only take a minute, I'll be quick alright?"
"I already told you Sammy, I'm not stopping." He repeats sternly. "Either use your diaper or stop your whining."
"You can't seriously expect me to— I'm not a child Dean!" Sam exclaims.
He can be as stubborn about this as he wants, Dean is not pulling over.
"Then why are you acting like one huh? Whining and squirming in the passenger seat isn't doing much to prove your point. If I can hold it until we reach our stop then you should be able to do so aswell, but if you're not capable then clearly I was right putting you in diapers."
That's enough to get Sam to shut up and go back to pouting, like a child. Which only further proves Deans point.
He discreetly watches as things get worse, as Sam's expression gets more and more pained and when they drive over a pothole he yelps before shoving a hand between his legs. Dean can only wonder if he leaked or not. Sam's face burns with shame when he manages to get himself back under control and he removes his hand.
Dean truly is a kind big brother for not commenting on that. A saint even.
It doesn't take long until Sam is whining again and Dean wonders how stubborn he's going to be about this. If he'll hold it to the point of giving himself an infection. That'll only give Dean an excuse to keep him in diapers for longer and to take care of him.
"Dean," Sam whines out, getting Dean's full attention once more. "Please, I can't… I can't hold it much longer. Can we just pull over? I promise not to ask again. Just once, please De."
He looks pitiful enough that Dean feels mild guilt as he denies him once more.
"You already know my answer Sammy, you're only hurting yourself by being stubborn." Dean says, looking over at Sam with his best concerned expression. "You know I don't like seeing you hurt yourself baby, stop being stubborn. It'll feel good to let go."
He removes one hand from the steering wheel to put it on Sam's thigh in attempt to comfort him, to convince him to give in. "I'll clean you up right after. It's okay. Nothing to be ashamed of."
For the first time, Sam seems to consider it. His eyes are wet with unshed tears and from the look on his face Dean can only assume that the adult mindset that he fights so hard to keep is quickly slipping away. He only needs a little push.
"Come on Sammy, let go for daddy."
Sam doesn't want to fight it anymore, it hurts.
But it's hard to let go like this, even while wearing protection. He's never used diapers at this age and he's afraid that it won't work, that it'll soak into the car seat. And Dean will certainly be furious if he damages the Impala.
But it hurts, he wants to let go so bad and Dean is telling him that it's okay, if Dean says so it can't be all that bad.
His hand goes to hold the one on his thigh tightly and Dean squeezes back encouragingly. Sam closes his eyes because this way it's easier to pretend he's not about to wet himself on purpose in the car, he takes in a deep breath and lets his muscles untense and does his best to relax.
It's a bit jarring when the first drops leave him but he forces himself not to tense up again, thanks to his efforts it slowly turns into a stream and he lets out a sigh of relief as he lets go. There's a hissing sound that he hopes Dean can't hear. He can't remember the last time he's felt such deep relief letting go, but he usually doesn't hold it this long. It just keeps coming out. More and more urine soaking into the diaper, it feels warm. And some sick childish part of him finds comfort in the feeling of sitting in his own hot urine. It feels amazing and it just keeps coming it almost feels like it'll never stop.
Thankfully it does before he oversaturates the diaper with urine, the strong stream weakens before turning into a last few drops.
His eyes slowly open and he looks down at his pants to find them dry, he somehow expected there to be a mess even with the diaper. And he's only noticing now that the car has stopped moving, they've parked on the side of the road. He glances over to see Dean looking at him with a strange expression. "…De?"
"God Sammy, you did so good." Dean says before suddenly leaning in and cupping Sam's face to leave small kisses all over it making him giggle. "So good, I knew you could do it. See how easy things go when you stop being stubborn? When you just give in?"
Maybe if Sam were feeling any more adult he'd argue but all that he manages to feel now is joy at Dean's praise.
"C'mon let's get you cleaned up buddy, don't want you to get a rash." Dean says and leaves one last kiss on his forehead before leaving the car.
He moves around to Sam's side and opens the door for him and helps his baby brother out of his seat. He leads him to the backseat and instructs him to lay down. Sam doesn't object, he's all obedient and pliant now, Good.
Sam stays still as Dean peels his pants down, revealing the wet diaper. A small embrassed whine leaves him as he covers his face but Dean gently shushes him. He undoes the tapes of the diaper before pulling down the front and the sight makes him pause.
Sam is half-hard for some reason and the sight almost makes Dean faint. What has he done to deserve such a blessing?
"Did it feel too good to let go baby?" Dean asks and it's adorable when Sam looks down in confusion before noticing what he means and blushes. "Don't worry, I'll take care of it Sammy."
Sam doesn't have the time to wonder what exactly Dean means by this because seconds later Dean is taking his cock into his mouth and a strangled sound leaves Sam.
"Wait, it's dirty—" Sam tries to say and his attempts at moving away are useless as Dean simply pins him down.
"I could never find any part of you dirty Sammy." Dean says, and it sounds so brutally honest that Sam doesn't manage to doubt him.
The wet heat of Dean's mouth around him feels incredible no matter how many times they've done this, Sam can never get enough of it. He blows him down to the base of his cock, an impressive feat considering the size. It feels so good, and he doesnt bother suppressing the small moans that leave him as his big brother pleasures him.
Dean has always been an expert at making Sam feel good so it doesn't take long for him to get close to his climax, he barely has the time to warn him before he's cumming into his mouth. Dean swallows it all down and sucks him off all the way through his orgasm until his legs are shaking and he's whining.
"You're perfect Sammy." Dean says as he pulls away; Sam guesses that Dean must've gotten himself off aswell while blowing him when he notices cum on his hand before he wipes it away.
He feels slightly bad for not doing anything for Dean in return, he'll have to make up for it later.
All Sam can do is bask in the afterglow as Dean cleans him up, he wipes Sam clean with wet wipes and throws the dirty diaper away into a plastic bag. He puts on Sam what must probably be baby-powder before putting a fresh diaper on him and taping it up. It feels soft and comfortable.
After that he pulls Sam's pants back up hiding the evidence. "C'mon lets get back on the road babyboy, up and at em."
With a small groan Sam sits up before exiting the backseat and moving to sit back in the passenger seat at the front, Dean joins him and they start driving again with no other sound than the one from a rock song on the radio present.
Dean looks oddly satisfied, Sam doesn't comment on it. Instead choosing to lean his head against the window, he closes his eyes and it doesn't take long for him to doze off.