ooo 🤔🤔 interesting question!
osei has an older brother named aaron. he’s the same age as baxter, so he’s not really prominent in his life, especially since he leaves to study law all the way over on the east coast at harvard. he also has a little sister on his father’s side named hyun-yae (osei’s character is very much not canon compliant to the olnf story) who eventually realises they’re non binary and go by they/them.
i remember osei being related to user sambaeksam’s mc yujin (or yunjin i dont remember ) since they had the same last name, but since i changed that i suppose they’re distant cousins now? osei’s dad’s last name is still kim, but idk.
as for relatives like cousins and such— osei has a few cousins from his dad’s side, and a grandmother + two aunts. on his mother’s side he has four aunties and an uncle, and quite some cousins.
yu-ming is more canon compliant and is an only child, but does have some cousins back in china. she has two uncles and four cousins.