I was looking at a 2008/9 SW Insider magazine and they were talking about a video on Youtube or somewhere from a 1980 Today Show interview with Mark Hamill and Harrison Ford. They mention that Hamill hinted at Luke's sister in that interview. That means Lucas was thinking about that during or before filming ESB. That would fit with him calling to Leia at the end.
ESB seems to conflict with itself. It gives 3 bits of evidence: The awkward kiss, "No, there is another," and the call for help at Cloud City.
The movie was written & filmed over 2-3 years but the final edit was done at the end. All 3 of these bits, including the kiss, survived the edit. I don't see that getting through unless the sister DID exist by that point, and yet Leia was NOT her yet. This also fits with Gary Kurtz's memories that the earlier form of ROTJ had Luke leaving at the end to search for his sister. (Kurtz was not involved in ROTJ at all. But they had discussed the framework of it during ESB, because ESB was ending on a cliffhanger and Kathleen Kennedy was not in charge.)
The ESB evidence is almost enough to be conclusive that Leia was NOT the sister yet . . . but then Luke Force-communicates with Leia on Cloud City. I dunno what to make of that.
I could picture George wanting to make Leia Force-sensitive even though she's not a Skywalker. I could also picture George hand-waving the kiss and saying "It's no big deal, they don't know they are siblings yet."
IIRC Yoda's "No, there is another" line was partially stuck in there just to raise the stakes for the Luke/Vader duel. It weakened Luke's plot armor. He could be defeated and the good guys still had another path to victory.
Last edited: