Being a very big Star Wars fan, maybe I can shed some light on this.
Lucas did write the story on paper, maybe not in novel form, but he was extremely familiar with the vision he had for the basis of the story. Heck, is wasn't Skywalker at first but Starkiller. But I believe that he knew before hand that Leia and Luke were brother and sister, however, the general audience didn't. And to keep with the drama of a possible relationship, he kept them guessing until the end of Empire Strikes Back when the infamous line from Yoda is announced "there is another" and everyone and their mother was trying to figure out who the "other" was. I remember, cause I guessed Han.
In the third movie, it's explained by Obi-Wan that they were separated at birth and that it was imperative that the Emperor (Palpatine) knew nothing of this because he was much stronger in the force and would have known there was a relationship between Vader and the children. This shows there was a limit to Anakin's (Darth Vader's) powers. How could he know something when the other party had no clue either? To further that, Vader showed no clue that Luke was his son until the line "I am your father" in ESB. My theory on this since I didn't read the books was that possibly Obi-Wan told him he had a son right before he was killed by Darth in SW. I don't know. I wish I would have read those books and maybe I should to find out.
As a person grows in the "Force" they become more aware of their surroundings and loved ones. Leia mentions that "she knows and has always known" Luke was her brother in some sense, but she doesn't know how she knows. I've read further on from the movies with extended versions of the storyline from different authors. They explain that Leia's gift of the Force is more emotionally driven and she has very good perception of people's feelings rather than what their course of action will be. She struggles with a light saber, yet can sense the rage and anger or fear in someone very quickly. So for her to have known her mother being "very beautiful but very sad" signifies that her bond with her was an emotional one growing with the force inside since birth. I hope that made sense.