This is a bl right? So why does he call him brother? They better not be blood related

No I think they just tried to translate the "hyung"

based on other manhuas that i have read younger people adress older people as brother or sisters even if not related

I think after being on this site with tons of asian stories you should know by now that saying brother/sister doesn’t automatically make them actually sibling and that it can just be a word used to call someone older than them lmao