
The og is “hyung” means “brother” but not literally brothers sometimes friends, lovers etc use this term
Male —> older male “hyung”
Male —> older female “nona”
Female —> older male “oppa”
Female —> older female “unni”
In which all of the above could mean literally brothers sometimes friends/sister but it’s not it’s honorifics.
I hope you got what i mean English is not my first language

It's not about friends/sisters. It's just calling older people with honorifics. It's true they mean older sister/brother, but most Asian countries use this to respect older people in conversation, whether they are blood-related or not. (Like MUST use)
(It's just Western countries don't care much about honorific speech while Asians are really set on it, that's why there are problems when it comes to translation)
why did sayoung call mc as brother?
are they siblings?