Well, they *became* twins, but in the early scripts of what became episode IV, Luke was stated to be 20, Leia 16. I saw Star Wars the first time in '77, and there is *no way* that I believe that the whole arc of stories was in George's mind at that time.
On Harry Potter, I can believe that she had a pretty good idea of the final confrontation between Harry & Voldemort, but not so much the "how to get there" from book 1.