[ashesatdusk]

Ace and luffy not being blood related isn't that surprising. I don't know about japanese in culture, but asia especially in places where nuclear family (south asia, southeast asia, china) is a recent phenomena, anyone who lives with you is considered a relative of somesort even if they are not blood relatives. Sanskrit based languages which buddhism is derived from, the honorifics used with older brother/younger brother is the same for anyone of same status (e.g. cousin that lived with you), you also frequently adress your parents close friends with honorifics of aunt/uncle (though it may be a more distant related).