So are they siblings? Or jin is just an ex-subordinate of that yellow eyed daddy?

Pretty sure an ex- subordinate and had some type of relationship

Oh the reason it said "brother" is probably cos of some honorifics that translate to brother in english but doesn't necessarily mean sibling in the way you think

Pretty sure his "hyung" and they're mafia so it is normal for them to use brother, sister like a honorific use like japanese "aniki, aneki" in yakuzas.