To be honest, Lucas was pretty much winging it too during the making of the Original Trilogy. The whole concept of Darth Vader and Anakin Skywalker being the same person was cooked up during the making of THE EMPIRE STRIKES BACK, and likewise, Leia being the sibling of Luke between EMPIRE and RETURN OF THE JEDI.
Lucas just got lucky that it all worked out, and fans were more forgiving of these things back then as the sensational spectacle of a STAR WARS movie during the 1970s and 1980s made all other considerations kind of moot. Moreover, there were no earlier STAR WARS films for comparison.
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