As of "The Empire Strikes Back", Leia wasn't even Luke's sister. That movie still had the developing love triangle between Luke, Leia and Han, and Yoda didn't mention the Skywalker name when he told Obi-Wan Kenobi that "There is another."

I think that George Lucas planned to make Star Wars movies beyond "Revenge of the Jedi" at this point, and he had an idea for another Jedi who survived the purges. When you're talking about the original trilogy, you have to remember that Lucas was writing on the seat of his pants. He often made last-minute changes to the script, and he considered some changes that would have dramatically altered Star Wars as we know it. Luke could have fallen to the dark side. The Death Star could have been a planet. Han Solo could have been a greenish hulk or a trouser-wearing space gerbil. Peter Cushing could have been Obi Wan Kenobi!