CO said:
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.

Well, according to zombie's book, in the first draft of Empire, Luke talks to the ghost of his father on Dagobah, who tells him that he has a twin sister who is training on the other side of the galaxy. However, in later drafts, the sister (and father meeting) plot points are dropped. But Yoda does mention, "Now we will have to find another" (eventually becoming, "No, there is another"). The sequel trilogy was supposed to feature the other as the central character supposedly, but there was no hint as to who this other was supposed to be. According to zombie, when ROTJ came around, and Lucas decided to drop the sequel trilogy, he still had to have the other issue be addressed. Knowing he couldn't introduce a new character, he merged the other idea with the old Jedi sister idea, and the merging resulted in Leia becoming the other and Luke's sister.