AN: Written as an audition for the HPFC fanfiction F A C T O R competition. I don't read many stories featuring Regulus, so this is my version of him, and if he doesn't match with most people's, my apologies, but this has always been my interpretation.
Sirius and Regulus have an odd sort of relationship. They've always been a bit peculiar, simultaneously closer than most yet more distant.
The first thing Sirius can remember is leaning over a crib containing this little bitty boy that his Mum says is his brother. He can't believe anyone could ever be so tiny. He feels an instinctive need to protect this defenseless little munchkin.
At the same time, when his mother tells him to be careful, that "Little Reggie" is fragile, Sirius feels a bit resentful that his Mum will be careful with this boy, but Sirius isn't considered "fragile" anymore.
It stays like that until the end – there is always a part of Sirius that resents his brother, because, as Regulus likes to state as often as possible, "Mum likes me better!"
Still, the protective instinct stays, too. Sirius tries, always, to shield Regulus from the worst of everything. He takes the fall for everything Regulus ever does, and the gap between them widens as their Mum begins to believe in Regulus's perfection and Sirius's constant failures.
Still, Sirius always made sure that Regulus never had to face anything bad, anything that could hurt him, and maybe that was his failing. Because of him, Reg grew up in this perfect, fairy-tale world that was unrealistic at best and psychologically damaging at worst. He thinks maybe that's the reason why now Regulus can't believe him when he says this is wrong.
Sirius knows that the friction between them has been growing for years.
He knows that he and Regulus have been traveling in opposite directions, even before that. But he wishes it could be different. He wishes he could show Regulus why he's wrong, why Sirius is so convinced that he's right.
Remus knows that he was the wedge between Sirius and Regulus.
He knows, and he hates it. But Sirius doesn't blame him. Sirius says that he and his brother, they share the same volatile temper. Sirius says it was only a matter of time before they exploded.
Regulus is snooping. The letter is brief, innocuous, one of many. Sirius still wonders how he picked it out from the rest, how he picked that simple sentence out from the masses and masses of correspondence between the two of them over the years – the correspondence between all of the Marauders over the years. He found a miniscule phrase, really, that would've meant nothing to anyone else, nothing to anyone who didn't know Sirius so well. But Regulus knows Sirius's rhythms. So when he reads the line, "I don't need my lunar calendar to know what tomorrow is. Wish I could be there," Regulus understands. He doesn't even need to check to know that the times when Sirius is most agitated always line up with the full moon. He doesn't need a calendar to know that Remus Lupin is always sick on and around the full moon. He just knows. And when Sirius comes in, all Hades breaks lose.
"What're you doing in my room, Reg?" Sirius's voice holds less accusation than it probably should, given the circumstances. Regulus whirls around, holding the letter in his hand.
"How can you stand it?" Regulus asks, true puzzlement there, but accusation as well, more than even Sirius's, whose is justified.
"Stand what? And why're you snooping in my mail?" There is anger in his voice now as Sirius realizes something is wrong.
Ignoring the latter question entirely, Regulus clarifies, "Stand being friends with a monster."
Instantly, Regulus finds himself pinned against the wall, his brother's hand around his throat. "Take it back," Sirius demands darkly.
"Take what back? The truth? The fact that you're friends with not only a blood traitor, not only a pair of half-bloods, but a filthy little subhuman monster?" Sirius's hand tightens, and Regulus can no longer breathe. He thrashes, kicking out with his legs until a blow lands on Sirius's shin and Sirius lets go, dancing out of range.
"Remus is not a monster."
"He's a werewolf, Sirius! A dark-creature, a half-breed!"
Sirius's wand is suddenly at Regulus's neck.
"One more insult comes out of your mouth, and I swear I will hex you straight into next year." Sirius's voice is dead even and deadly serious.
Regulus sneers. "You wouldn't dare. Mother would kill you if you so much as laid a single jinx on me, and you know it."
"You think I give a crap what Mother thinks?"
"You ought to."
"Oh, don't even try to pull that holier-than-thou crap, Reg!"
Regulus simply shakes his head. "There'll come a day, Sirius, when you figure out that all of this," he waves his hand airily, gesturing at nothing, "this teenage rebellion stunt, or whatever it is, when you find out it isn't worth it. And you'll come crawling back, only there'll be nothing left for you to come crawling back to, because you've shoved everyone away."
Sirius stares in disbelief. When did his brother become someone he couldn't recognize?
"This isn't some, some teenage rebellion stunt, Regulus! This is me knowing what's right and what isn't, and this, this crap they've been spoon feeding us since we could listen, this isn't! This pureblood supremacy is all about shutting people out and killing them off because they're different! That isn't right, Reg! I see it, why can't you?" By the end, Sirius's voice is desperate, desperate for his brother to believe what he knows to be the truth.
But Regulus shakes his head. "You're wrong, Sirius. Family is what matters."
"Not when your family is a bunch of raving lunatics! That's when you've got to find your own family!"
"I'm sorry, Sirius. I don't know how you got so wrong, but you are." Regulus stares at his stockinged feet for a moment before adding, "You know I'll have to tell Mother."
"Regulus, you can't!" Sirius pleads.
"I have to. She has to know."
"But she'll tell everyone!"
"And they should know! There's a reason there's a registry for these kinds of…" Regulus stops mid-sentence at the look on his brother's face.
"Remus is not a monster," Sirius murmurs. "He's one of the kindest people I know, and he doesn't deserve to be judged for something he has no control over. He's on the registry, Regulus. But that part of the registry is sealed. Please." The word is not pleading. It is half-intense, half-resigned, and dead even in tone.
"He's a half-breed, Sirius, not worth your time! I have to tell her! She'll rid you of this nonsense."
Sirius pales, understanding the veiled threat.
"It's not nonsense! You don't know him!"
"I don't want to. Unlike you, Sirius, I don't associate with those less than dirt."
And Sirius snaps.
The first hex undoubtably comes from Sirius's wand, but after that it becomes impossible to tell. Spells fly all over the room, leaving chaos in their wake. The two brothers are a close match, always have been, but in the end it's Regulus left lying on the floor, still in unconsciousness as Sirius packs his things for the last time.
"I'm sorry, Reg. I tried so hard to save you," Sirius murmurs to his brother, before muttering, "Obliviate," and walking out the bedroom door for the last time for over twenty years. He walks straight past his mother in the parlor and straight out the door.
Sirius is sixteen-years-old. It is the the last time he ever talks to his brother. It is the last time he ever sees his mother alive. He never has another conversation with his father.
He walks straight out of their lives.