#long analysis sorry. esp because its about canon asdfghj

[starcrossedbrothers]

i find it so interesting that Sirius tells Harry that he found out about Regulus' defection after his death, because it doesn't actually make a lot of sense. all the circumstances surrounding Regulus' death are secret--the existence of horcruxes, voldemort's plans, Regulus' plan and decision--to everyone but Kreacher, who we know kept that secret until he told Harry 18 years later. (and if it isn't explicit, it is really likely that Harry was the only one he told, especially because he only did so on Harry's orders. otherwise, Kreacher probably wouldn't have had possession of the locket). Regulus' body is never found so considering the secrecy, it's likely that people only know he's dead from the tapestry of the Black family tree, which would have recorded it automatically with his death.

but. Sirius specifically says that he found out Regulus defected after his death. even though nobody would have known for sure. which begs the question: who told Sirius that and why would they tell him?

the most obvious answer is Kreacher but again, I don't think that makes sense. sure, Sirius could have forced him to tell, but if that was the case, why would Kreacher still have the locket? why wouldn't Sirius take it? and, even more than that, why wouldn't he know the whole story? Sirius believes Regulus' actions came from a place of fear and cowardice, but Kreacher notably calls Regulus brave, which doesn't work with Sirius' version of events, so it's unlikely he got it from Kreacher.

from what i'm thinking, there are two versions of how this goes:

The Rumour Mill: Regulus goes missing and, for one reason or another, everyone believed he defected. maybe it's because the Order didn't know how he died, so it couldn't have been one of their people/in a battle. maybe everyone looked down on him because of his age. maybe he was just a really bad death eater. maybe one of the death eaters falsely claims to have been the one to kill him on voldemort's orders. maybe something else i'm not thinking of. whatever the reason, this theory spreads like wildfire, and despite there being no evidence, Sirius chose to believe it. which says a lot. even after everything the brothers went through, and everything Regulus did, Sirius chose to believe he died a coward rather than a murderer. that he died, maybe not trying to do the right thing, but trying not to do the wrong thing.

Somebody Told Him: Regulus goes missing and no one has any idea what happened to him. not the death eaters, not his family. maybe it's advertised, maybe not. but eventually, someone in the family notices that the tapestry has recorded his death. (this is likely walburga i think--Kreacher probably wouldn't want to draw any attention to himself by claiming to know anything about Regulus' death, since she could order the info out of him, and Orion might have already been dead since he dies the same year as Regulus). so Regulus is now declared dead and someone believes he defected, believes it enough to tell Sirius, disowned and estranged as he is. it couldn't be Andromeda. it wouldn't be Walburga. but it could be Narcissa. Narcissa with her death eater sister, death eater husband, little death eater cousin who goes missing and she tried to work out what happened. she doesn't succeed, we know that, but maybe she gets enough info about Regulus before his death, maybe even from interacting with him and seeing how he was, to believe he defected. and despite Sirius being disowned and estranged, she tells him. maybe from a sense of obligation. maybe because she thinks he deserves to know--she's a sibling too. and he chooses to believe her.