Who is your relative and what that relationship means differs between cultures (and sub-cultures). But there are always rules, and with all rules there are exceptions. The typical one in English-speaking cultures is an adult who is not a relative by blood or marriage who is referred to as Aunt or Uncle.
Anthropologists refer to this as fictive kinship. I've done a bit of this, most recently when I decided that my uncle's partner was my aunt. Nice and neat, right? Anyway this has opened a new can of worms, as does this mean I've just gained a veritable hatload of fictive cousins, some of which I've never met? More relatives! Or just people related to my aunt? I don't know.
 Godparents get this quite a bit.