Sirius had always been jealous of Regulus. Perfect Regulus. The favorite, the son that his parents always wanted and he could never be.
He had tried, at first. His parents' fists were softer and their smiles were brighter when he pretended. So he dressed up and went to their parties. He was the polite black heir was expected to be. And he hated it.
But Regulus never had to try, he just was. Sirius never understood it. What did he have that Sirius didn’t? Maybe it was because he couldn't keep his mouth shut, couldn’t follow the rules, or just didn’t want to. He couldn’t keep who he loved or what he thought a secret, and Regulus could.
The day Sirius was sorted in house Gryfindor had been the end of his pretending. But under his breath, he had repeated ‘Slytherin’, ‘Slytherin’, ‘Slytherin’, anyway. When the hat shouted ‘Gryffindor!’, Sirius would never forget the feeling of dread that filled his stomach. He’d never forget his parents' faces when they had found out either.
After that day Sirius could no longer hide who he was, and he decided he was done playing his parent's game, trying to live up to the little brother that had always been better. He stopped being polite, he stopped following the rules, and he stopped trying to be someone he never would.
He left and became the opposite of what he knew his parents wanted for him. It wasn’t just for his parents, but Regulus too. The way he acted, the way he talked about his brother, he resented him. He had been confident, pretending he knew who he was, what he was doing, just so Regulus could see him, could see that he didn’t need that life.
It was a lie. He wasn’t confident, he didn’t like standing up to his parents, and he didn’t hate his ‘pratty stuck up little brother.’ Almost everything about the Sirius Orion Black that people saw was fake. Everything except Remus. That was the most real thing he had ever had, and no one could take that away from him.
Sirius had always wanted to be Regulus. He had always wanted his parents to want him, love him as they did Regulus. And for so long of his life he was mad at him. His anger was misdirected. He was young, he didn’t know what else to do with it.
But the thing was, he didn’t hate Regulus, he never did. He missed him. He missed when they were just kids, when they didn’t know any better, when they were all the other had.
But they were worlds apart. Maybe they didn’t have to be. Maybe in some other universe, Sirius would have listened to Regulus, accepted him, and saved him from his parents. Maybe they could have been close. Maybe they could have been brothers. But Sirius never did. Even if he decided he wanted, even when he realized he should, it was too late.