Well, Dick/Tim has a fairly stark power dynamic. Luke and Leia met as adults and interact as equals. (Also, the comics shipped them like mad before Lucas Jossed them, which probably goes some way toward normalizing the idea for anyone who read them.) Considering the attitude of fandom toward sex, it makes a lot of sense that emotional relationships are far more significant to whether stuff trips as incest than sheer genetics.

(Personally, I'm indifferent to most Batshipping - I don't see it, but it doesn't squick me - but Earth-2 Dick/Helena makes me twitch violently, and Johns went and made it angsty canon, AUGH. I mean, Wanda/Pietro as Ultimate canon is kind of fun, but only because it knows it's wrong and doesn't pretend to be healthy.)