@torzin if youre referring to the person serenia called unnie, it explicitly said unnie. and no, you cant use hyung and unnie interchangeably lol. unnie is strictly for when a girl is referring to an older girl.
I was watching the most recent ep of All the Butlers (Ep. 142) and the guestās brother said that he actually calls his older brother āunniā at home since their mom is interested in pure Korean and in pure Korean it doesnāt matter. Thanks!