obviously sokka is like, the platonic ideal of an Older Brother. and katara and azula are both suuuch Little Sisters. and it’s clear that toph and aang are both Only Children. but zuko.... does not have older brother energy. at all. so I think that means that lu ten, at some point, must’ve been around a lot, and was the older brother figure that both zuko and azula looked up to. and that’s why zuko is, in fact, the ultimate Middle Child.