I think in the past they weren't biologically related, sometimes calling someone 'brother' or 'sister' is just like a term of affection. Theyve been together since they were children so they could see each other as brothers. In the present they are biologically related.
They are distant relatives, but hyung or unnie is also a term of closeness/affection between friends who are of the same gender but slightly apart in age. It literally translates to brother or sister in english, but it's also used for unrelated people
Most asian cultures use familial terms for unrelated people to indicate closeness, like mom/dad for inlaws or best friends' parents, uncle/auntie for neighbours, grandpa/grandma for any old person regardless of relation etc
what the fuck is happenign ?!
are they siblings ? tf