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Originally posted by: ChainsawAsh
Actually, the sister thing was thought up on the fly during ROTJ. It was never brought up in Empire.
Not directly, but the whole scene where Luke's leaving Dagobah and Ben says "that boy is our last hope" to which Yoda replies "no, there is another".... - that's certainly a foreshadowing of the incest weirdness. Lucas was already losing what little storyline he had and was setting up for the next wave of downsizing the whole story. All that was left was to unnecessarily tie up a few more non-existent loose ends so that the story was even less of an outer space adventure from far far away.