Chad: That's a very good point. I think it's resolved then that Darth Vader didn't understand Leia also existed before ROTJ, yet Yoda's quote "No. There is another" has always confused me. Either in ESB that was already known that Leia would certainly be revealed as Luke's sister, or they're talking around someone else. I uncover the before be an ext believable.
No.At that allude he doesn"t even know Luke is his son and that"s what start his interest in recognize "Young Skywalker."
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He mirrors his surprise at finding out that Luke has actually a sister in Return of the Jedi when he states he watch Luke"s feelings are strong for his girlfriend – climate adds, in surprise, "Sister."
Wookieepedia states (when explicate the duel in between Luke and also Vader top top the brand-new Death Star):
Vader, who had actually been research Luke"s thoughts, uncovered that as well as Luke he likewise had a daughter: Leia Organa
However, no attribution for this assertion is given in the article.
To re-use part of mine answer come an unrelated concern Why do we listen Leia's Theme throughout Kenobi's death?:
At the moment of functioning of "A new Hope" (e.g. Prior to 1977) - it was NOT known to anyone (including Lucas) the Luke and also Leia are family. Due to the fact that Luke was Anakin Skywalker"s child from the beginning of ANH, that means Leia wasn"t supposed to be his daughter, till at least mid-work on illustration V.
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To be an ext specific, even very early Leigh Brackett"s 1978 breeze of "The empire Strikes Back" didn"t have actually that relationship developed yet - it was presented later (source).
Another related evidence for Luke and Leia not yet being associated was 1978 pre-ESB book that to be a sequel come "ANH" - Alan Dean Foster"s "Splinter that the Mind"s Eye". It develops the theme of romantic interest in between the two (presumably, in part, due to the fact that Foster and Lucas didn"t know if ANH would be a big hit and also if Harrison Ford would certainly be interested in the sequel need to the sequel be based upon "Splinter")