Apparently, Lucas said when in the planning of Empire Strikes Back that who Leia ended up with was "anybody's guess." And that Luke was more devoted to her. Quote from "The Making of Star Wars" featuring R2-D2 and C-3PO.

I always thought he had a little idea for Leia to be related to Luke, but it turns out I was wrong entirely. So, he's definitely rewitten at least SOME Star Wars history. I always thought there were some parts he made up and others he didn't, but this proves Leia's relation to Luke was a decision he hadn't had at the beginning - which, mind you, he insisst now was.