it's been a while since I read it, but i seem to recall an article from the Official ROTJ magazine (which I had when I was seven and read until it literally fell apart in my hands) where Lawrence Kasdan definitely says the Luke/Leia sister thing came up in story conferences for Jedi. I may have also read something like that in the Annotated Screenplays books.

not sure if the germ of the idea didn't fall into george's brain while on a plane, tho. perhaps whilst joining the mile-high club with linda rondstadt?