Obi-wan does say that Luke has a twin sister in ROTJ ("The other he spoke of is your twin sister"). If I could (gladly :D) only consider ANH and ESB, on the other hand...!
Speaking about Leia, any chance someone can do an ESB's fanedit to change "There's another" back to "We must find another one"? Then cut the whole twins' subplot in ROTJ, and that would make for my ideal OT :D