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Humans, Castiel had noticed, were strange creatures. Even if he was more in tune with his humanity than ever, having been a human himself, there were things they did that never failed to confuse him. He understood love and sex (thank you, Dean), personal space, the concept of privacy, lying, and he knew that a lot of the things they did were out of love and care for one another. But there were some habits he just didn’t think he would ever truly get.
One of those things was so-called sibling behaviors.
Castiel had siblings. He had a LOT of siblings. Some were like him, fascinated by humanity and all too happy to observe the lives they lived and protect them as Chuck had once intended. Others were jealous, callous, and as Dean would describe them, “dicks with wings.”
He’d obviously had his share of sibling fights, though it usually resulted in the death of another Angel. He was made to be a warrior, after all, and he was no stranger to battle.
This was different.
Since moving into the bunker, Dean and Sam had become quite domestic. They’d both had the opportunity to choose their own rooms for the first time and had picked rooms far enough down the hall from each other that they could still feel comfortable knowing the other was safe while still having their own spaces to relax. This was understandable. Privacy was very important to humans.
Some nights after dinner had been made and eaten and the brothers had spent time together, they would retreat to their rooms to decompress after a busy day.
And some nights, the brothers would do something strange.
“Hey.” Dean said, leaning against the doorframe of Sam’s room.
Sam sighed and took off his headphones, glaring at his brother.
“What, Dean?” He asked flatly. “I’m a little busy.”
“What are you doing?”
Sam rolled his eyes. “Reading. Leave.”
Weirdly enough, Dean just stood there. He shuffled on his feet silently, adopting a strange stance and tiptoeing into the room.
“Oh my God-“ Sam groaned and leaned back in his chair. “Dude. We are not doing this.”
“Wait wait just look at this-“
“DEAN.”
Castiel watched from the hallway, head tipped to the side curiously. Was this some kind of ritual?
Dean grinned obnoxiously and squatted, scooting further in the doorway.
It looks could kill, Cas thought, Dean would be dead.
“I swear to fucking God, Dean. GO AWAY.”
“C’mon Sammy I just wanna-“
“OUT.”
To no one’s surprise, Dean did not leave. Instead, he stood back up and did a spin on his heel.
And then farted.
Irritation rolled off Sam in waves as he grabbed a piece of paper, balled it up, and hurled it at his brother.
“You’re DISGUSTING. Get OUT OF MY ROOM.”
Dean’s insufferable giggles filled the air as he finally stepped back out of the doorway, doing a strange little dance.
“CLOSE MY DOOR-“ Sam yelled after Dean as he ran back down the hallway.
Dean, of course, did not close the door.
Castiel had asked what that was about not long after.
“Oh y'know,” Dean had said, snorting as he heard Sam’s door slam down the hall. “Just sibling stuff.”
“Angering your brother is… Sibling behavior?”
Dean nodded. “Yeah. It’s like… an instinct I guess.”
“Instinct.”
“To annoy my brother, yeah.”
Cas shook his head. Humans were certainly strange creatures indeed.
—————
The next incident was even more bizarre to Cas. This time it was Sam’s turn to be obnoxious.
The brothers were in the kitchen, Dean cleaning up after making his famous burgers. Dean had used up the last of the paper towels and set the empty cardboard tube on the counter as he crouched down below the sink to grab a second roll.
All conversation stopped as Sam stared at the empty roll, the turning gears in his mind almost visible as he considered the empty paper towels.
“Sammy?”
Sam suddenly reached forward, grabbing the empty roll and rearing back before coming down as hard as he could on the back of Dean’s head with the tube.
Dean yelped, grabbing the back of his head and whipping around to face his brother.
“What the hell, Sam?”
Sam nearly doubled over with laughter as he reached over to smack Dean in the arm with the tube again.
“SAM.”
Dean grabbed another roll to defend himself and it turned into an all out paper towel sword fight, the brothers chasing each other around the kitchen as sharp smacks filled the air.
This went on for at least 10 minutes before Sam finally ran to his room and shut the door, his laughter floating through the hallway as Dean pounded on the door.
Strangely enough, Dean didn’t actually seem to be upset. Maybe annoyed, but he was almost laughing? Even covered in red, paper towel tube shaped marks, he was laughing.
“I don’t understand.” Cas said later when the three of them were sitting at the table in the library.
Dean quirked an eyebrow as he looked up from his laptop. “Understand what?”
“Earlier in the kitchen. Was Sam angry with you?”
Sam snorted and shook his head. “Nah. Dean’s not the only one who can be a jerk.”
“Bitch.”
Sam squinted at his brother and rolled his eyes. “Sometimes you just get the urge to beat your brother senseless, y’know?”
Cas’ brows furrowed. “No.”
“It’s a human thing.” Dean leaned back in his chair. “The Cain Instinct.”
This was just getting more confusing by the second.
Sam shook his head. “Not in the biblical sense. I don’t actually want to kill him. Well… not right now anyway.”
“Hey!”
Dean leaned over to smack his brother’s arm.
Cas cocked his head. “So sometimes… you just feel the urge to enact violence upon your brother.”
The brothers nodded in unison.
“Yup.”
“Huh.”
While he understood most things about them, Cas was sure he would never fully understand humans.
———
For the first time in a very long time, Dean and Sam had a fight. They hadn’t come to blows but it definitely wasn’t pretty.
The yelling rang throughout the bunker as the two of them argued, ending when Dean finally stomped down the hall to his room and slammed the door.
Cas honestly wasn’t even sure what they’d been fighting about. He was pretty sure it had something to do with Sam stealing one of Dean’s AC/DC shirts and getting a stain on it but he wasn’t completely sure. How something so trivial has devolved into verbal abuse, he truly had no idea.
An hour later, as he sat on Dean’s bed and rubbed circles on his back, Dean shook his head.
“I’m hungry.” He muttered.
Cas was about to suggest he bring them something when his partner stood up suddenly and headed for the door. Cas followed, unsure whether or not he should be asking what Dean was doing or continuing to offer support.
To his surprise, he wandered down the hall, stopped at Sam’s door and knocked. When there was no answer, he knocked again.
“Come in.” Sam said quietly on the other side of the door.
As Dean opened the door, he leaned against the doorframe. Cas half expected him to do something stupid again like squat and yell but strangely enough, he didn’t.
“Hey.” Dean’s voice was surprisingly soft as he watched his brother warily.
Sam glowered from his place at his desk. “Hey.”
The two of them merely stared for a moment, almost as if sizing each other up.
And then Dean spoke.
“Wanna go get a burger?”
Cas half expected the door to slam in his face but… that didn’t happen. The silence stretched for what seemed like hours before Sam finally stood up.
“You’re driving.”
What in the goddamn-
“Wait.” Cas said as the two of them hurried to the garage, laughing and grabbing their coats as if nothing had happened. “Were you not just fighting?”
“Well yeah,” Dean grabbed his keys and hurried to his Baby. “He pissed me off.”
“But we’re hungry.” Sam finished.
“Exactly.”
As Cas sat in the back seat of the Impala and listened to the brothers argue about where to eat, he couldn’t help but wonder.
Was this part of The Cain Instinct too?
——————
In the end, Cas decided he would probably never understand the Winchester brother’s behaviors. It was clearly something you had to be raised with a human sibling to understand.
As he watched the two of them wrestle on the floor of the library, Dean on top smacking Sam with a pillow that he seemed to materialize from nowhere purely out of a lust for violence, he decided it didn’t really matter that much.
The brothers were as close as ever and that’s all that really mattered to Cas. He didn’t need to understand.