At first, Sirius is Walburga and Orion's favourite child. Regulus really is just a spare. Up until Sirius gets sorted into gryffindor. Walburga takes her disappointment and anger out on Regulus. Orion is so angry, he doesn't talk for days.
Shame, really. Sirius is the one who has the qualities of a proper heir. He's dominant, confident, he has charm. Not Regulus. Regulus is not heir potential. He's quiet, he cries, he gets messy when he eats. He wasn't ever meant to be a heir, Walburga thinks.
So she spends that whole year with Sirius away trying to make an heir out of him. At every minor mistake, he gets beat. And when Sirius gets back from Hogwarts that year, he thinks Regulus grew to hate him like the rest of them. That's why he thinks Regulus is like the rest of them.
Regulus plans on telling him that that's not true over the summer, but Walburga uses legilimency and then forbids him from talking to Sirius.
Orion watches all of it. Maybe he should've ran when he was young. This really isn't ideal.
At Hogwarts, when Regulus gets sorted into Slytherin, that just proves that Regulus is like the rest of them, Sirius thinks. Regulus begins to think it too.
They have an off and on relationship at Hogwarts. Mostly off.
Sirius and Regulus Black stopped talking altogether after Regulus got the mark. Deep down, he didn't want to. Walburga knew that, obviously she did. Her favourite spell. So she started being even more cruel to him. Orion too. The mark made Regulus lose all hope in ever being on good terms with his brother.
Rgeulus Black screams. Nobody hears him, he's underwater. Not Barty, not Evan, not Sirius. He doesn't know why he's surprised. Even if Sirius did hear him, he doubts he'd try to help. That's what Regulus deserves, he thinks.
Regulus Black is dead. When Sirius hears, a small part of him dies. He tries to pretend it's fine. It's not. His brother's gone and he can't come back. Barty Crouch jr is the person who tells him. Sirius doesn't think he's ever seen him weep before.
Sirius Black is dying. He's falling through that damn veil. Images flash before his eyes. James, Remus, Mary, his friends. And in the final moment, between life and death, he thinks he hears Regulus laughing.
Sirius Black is dead. Regulus Black is dead. They could've been great together. A wasted opportunity.
You can't force the stars to align when they've already died.
