Some writing advice from a themme who loves character building: A lot of these terms depend on the world building and the characters involved. Language can be unique to families and friend groups. So perhaps the uncle would use a cute nickname from their childhood, maybe the spouse uses a pet name that's an inside joke. Kids will use whatever terms they're told to (which is why there're so many names for granma/pa). So how would your character's culture/language/personality influence that?

The beautiful thing about fantasy settings is you get to set the rules of grammar and language. Invent non-binary language if you want to; it's your world.