This is the one time that I am going to defend Lucas on his older quotes about the saga now. In one of the documentaries in the recent DVD releases Lucas says, "It was always about these twins, and their father."

Now in that way, I do believe him, cause I think he did have a general idea about that, but he never specifically says Luke/Leia are siblings the same way he lies about Darth Vader being Lukes dad or the saga was always about Anakin.

I think in ESB, the other was Lukes sister, just not Leia at the time, hence why Luke & Leia smooch at the beginning of the movie. I think if Lucas decided to make more movies with Luke, Leia and Han rather then a trilogy, in the next movie they would explore Luke meeting his long lost sister and he would train her in that movie to become a jedi. But I think Lucas wanted to end the series with ROTJ, they needed to tie up the 'other' plot point, and Leia was his only way of doing that.