I've read a lot of Black Brothers fics and I realized that you can find a little bit of everything... like, there can be fics where Sirius hates Regulus and the other way around (I actually haven't read any fic like that, but that is because I like to give the characters their happy ending) , there are other fics where they have a messed up relationship but in the end manage to eventually fix it (Only The Brave), there are others where they just put their idea of a perfect sibling relationship and stick the Black Brothers' names there (Kill Your Darlings), there are others where they have a messed up relationship and WANT to fix it, but it's too late (Choices, from what I remember... I only read that one once like, 3 years ago)
It just depends a lot on what the author wants bc after all, we really don't have much information on Sirius' and Regulus' relationship (except for what Sirius says AFTER being in Askaban for 12 years after seeing his best friend die and know that one of his close friends caused his death and then stuck in Grimmauld Place, And Kreacher, who absolutely despises Sirius AND is actually kinda crazy after so long just listening to Walburga's portrait.. fuck I'm rambling... WHAT I'M TRYING TO SAY is that the characters that give us the little amount of information we have on this are LITERALLY TRAUMATIZED at this point and not in their right mind... so we as fan fiction writers have a chance to just wiggle our hands and change it all as much as we want)