Lucas has changed his story on Star Wars several times. Whether it was one trilogy (to rule them all?), a three-trilogy mega-movie series, or the saga of Anakin Skywalker, the man has left many fans clueless; those who aren't, are angry or have just stopped caring.
I think that by the time he finished the original Star Wars, he at least wanted to keep going and had some idea of ESB. I also think that, even from the beginning, Darth was going to be Luke's father. I mean, his name's kindof a clue - I believe it means "Dark Father" in some language. But, Leia being Luke's sister? That was definitely a tack-on at the last minute to try and add some suspense and surprise. Splinter of the Mind's Eye proved it to me.
So, what do you think? How had he originally planned it out?