Chad: That's an extremely excellent suggest. I think it's settled then that Darth Vader didn't understand Leia even existed prior to ROTJ, but Yoda's quote "No. There is another" has constantly puzzled me. Either in ESB it was already known that Leia would certainly be revealed as Luke's sister, or they're talking about someone else. I uncover the before be more believable.
No.At that allude he doesn"t also understand Luke is his son and also that"s what starts his interest in finding "Young Skywalker."
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He reflects his surpincrease at learning that Luke has actually a sister in Rerevolve of the Jedi as soon as he says he sees Luke"s feelings are solid for his friends – then adds, in surpincrease, "Sister."
Wookieepedia states (when describing the duel between Luke and also Vader on the new Death Star):
Vader, that had been probing Luke"s thoughts, discovered that besides Luke he likewise had a daughter: Leia Organa
However before, no attribution for this assertion is offered in the short article.
To re-use component of my answer to an unassociated question Why do we hear Leia's Theme in the time of Kenobi's death?:
At the time of working of "A New Hope" (e.g. before 1977) - it was NOT recognized to anyone (consisting of Lucas) that Luke and Leia are family members. Since Luke was Anakin Skywalker"s boy from the start of ANH, that means Leia wasn"t intended to be his daughter, till at leastern mid-occupational on Episode V.
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To be even more certain, also an early Leigh Brackett"s 1978 draft of "The Empire Strikes Back" didn"t have that relationship establiburned yet - it was presented later on (source).
Anvarious other related proof for Luke and also Leia not yet being associated was 1978 pre-ESB book that was a sequel to "ANH" - Alan Dean Foster"s "Splinter of the Mind"s Eye". It develops the design template of romantic interemainder in between the two (presumably, in component, considering that Foster and also Lucas didn"t understand if ANH would certainly be a big hit and if Harrichild Ford would be interested in the sequel must the sequel be based on "Splinter")