They shared everything.
Well, not… everything. Like, there was stuff that was still private. Underwears, girls, secrets. But they shared a lot.
They shared clothes. It started when they were kids. In tradition of little brothers everywhere Sam wore Dean's hand-me-downs. Unlike little brothers everywhere, though, Sam didn't mind it one bit. Little Sam adored his big brother and felt just as cool as Dean wearing his clothes. Teenage Sam could see his family didn't really have all that money (or wardrobe space… or a wardrobe) and actually prefered saving his money on clothes to spend it on books.
Adult Sam had shared clothes with his brother for such a long time, he really didn't think about it. They didn't share everything, of course. After he turned 17, Sam's pants were way too long for Dean to borrow. Dean's (actually dad's) leather jacket was off limits, not that Sam would want it any more than Dean would want any of his 'ugly-ass shirts', as he put it. But Dean did use Sam's hoodies whenever he felt off-duty enough and Sam did steal Dean's military cargo jackets when Dean was out and he felt lonely.
And just as Sam once put socks on his brother's sneakers to wear them, after he surpassed his brother in height and shoe size, Dean also started wearing his brother's boots with the extra room. Their T-shirts and plaid shirts were passed around in each other's duffels so many times they forgot which one belonged to whom after awhile. Socks were common territory, they were all from the both of them. Which, of course, led to them always thinking they had more clean pairs they actually had and blaming the other when they ran out.
They shared money. When they ran credit card scams, they always got two cards out of each deal, both with the same amount of credit, one for each. When they hussled pool, no matter who the player of the time was, he'd get the night's amount and split it in two.
When they were kids and dad left for a job, Dean was the one responsible for the money because he was the oldest. But dad did give his sons allowances whenever he could, even if it was just a few coins for the week to teach them the value of money. No matter how much it was, it was always evenly shared. "Because you are brothers".
When Dean got his first job, as an assistant in a garage when he was 13 (but lied he was 15) and they were staying in town for an entire month, he split his first paycheck in two and gave half to his brother. He didn't even think about it. It's not like he was working for himself, anyway. He'd much rather sit on the room and watch TV all day, but they needed the money. Dad was a hunter and hunting was no pro ball carreer.
When Sam was in high school and working at the library, a classmate that worked with him looked as if he'd grown another head when he gave half his paycheck to his older brother. Sam just looked puzzled at the questions and shrugged as he answered "We're brothers".
They never shared a girl, really, cause… gross. But once, when Sam was 18 and Dean was 22, a girl from Sam's high school senior class dated them both for a couple of weeks. Jenny Clark. They both found out separately and they both stepped back for his brother, breaking up with her. Jenny ended up with neither. They never told each other about it.
They shared blood. Not only in the fact that they were born to the same mother and father, but, literally. Dad taught them both how to make a DIY's blood transfusion, in case of an emergency. Once, when Sam and dad were injured in a hunt and Dean was left to take care of them, Dean almost bled himself dry transfusing to an 15 year-old Sam. John woke up from the bed his son put him to sleep with an uneasy feeling, only to find Dean passed out on the floor and a long line draining him of his blood in favour of his brother.
Many years later, one after John's death, Sam found his brother being bled out by a Djinn. Adrenaline kept Dean on the fight long enough to save Sam's ass again and the girl next to him, but as soon as they stepped out of the building, he collapsed to the blood loss. Sam left the girl at the hospital and took his brother to their room, because he didn't need doctors for this. He set the transfusion up and watched numbly as the moment when Dean had enough and Sam would be the one suffering from blood loss came and went. His mother and father were dead, Jessica was dead, his brother was going wild trying to protect him from some evil plan they seemed unable to understand, let alone stop. He let the blood flow, closed his eyes to blessed darkness. And was jerked awake by a fierce slap across his face and Dean's furious eyes.
What they shared most, though, they didn't speak of, not ever. No chick-flick moments, not between Winchesters. When one slipped, the other had the barb at the tip of his tongue to put the appearances back in place.
What they shared most was love.
They didn't speak of it, not ever. Because it was laid out for them to see in everything else they shared.