it is a cool choice to write regulus as being permanently in sirius' shadow and would grate against that. i understand that some writers make this the basis for the j/egulus dynamic of "i am deeply in love with my brother's best friend but It Cannot Be because my brother and i have a weird relationship", but it's hard to write that without changing a lot of their personalities. while i can see how good writers would take a mile if you gave them an inch of inspiration, i never got on the j/egulus train simply because i was rotted by a more compelling ship route to take
namely, that regulus would get with snape in order to get back at sirius.
your baby brother falling in love with your amazing best friend? ew, but also, you can't fault his taste. your baby brother doing the nasty the worst supremacist scum in the british isles, on the other hand? vile! disgusting! treacherous! sirius would absolutely go nuts if his baby brother debased himself in such a way– which is exactly why i think regulus would go for it. it is the sort of slytherin-flavored vengeance that befits regulus more than softly angsting under dramatic light in a corner
and the reason why sirius would freak is for a multitude of reasons! regulus is a black– pureblooded, high-class, honor above all. sirius' value system of honor would not accept that his own blood would associate with hypocrite supremacists such as snape (though sirius himself is a hypocrite through regulus' eyes because he loudly goes against their traditions but wants to effectively manage and control someone within the institution he left)
perhaps, years later, sirius might still wash out some of regulus' blame— his idiot brother, too soft to know when his lover (shudder) was joining a supremacist organization
but snape is an effective knife for regulus to hold against his brother simply because snape is the sirius-that-never-will-be
because snape is accepted by the same family that callously disowned sirius! he breaks bread with the black cousins! joined the same after-school traditionalist club! quite possibly (i don't remember if it was canon) also named draco's godfather! sirius hated that this greasy git fit better into his family than he ever would
(and had he lived longer, he would be simmering with envy of how effortlessly snape still swans in their circle despite being proved a LITERAL SPY. he is stuck and tied in his house but snape, of all people, gets to have afternoon tea with bella and cissa)
and snape hates sirius because he is effortlessly powerful and cool– even by muggle standards!! it is hard to be cool by muggle standards when there's so much going on in that world but somehow, this posh bloke steps in london and immediately becomes a café racer. he transfigures his robes to mimic muggle clothing and still looks fashionably rockstar handsome instead of looking like he came from a multicolored freakshow circus like every wizard trying to fit in the muggle world
sirius bought a nice flat in the richer side of london right after graduating, without looking for a job. he knows what the current movies are, has paid for all of his friends to watch it together, and is in tune with muggle life the way he would never be
(the day snape found out that sirius, who was raised on enchanted concertos and magical singing kitchenware, bought a pregnant lily potter VIP seats to an artist's concert— the artist who they both first heard on a rickety radio player in his mother's house— he flipped his shit and applied for his second masters)
they are soooo deeply jealous of each other's ability to fit in. and it would make for a really good dynamic. all three of them are playing each other like fiddles