Also, it's pretty clear that Luke is trying to bang her in Empire. He says he is in love with her in an early draft, in the final draft Leia kisses him on the mouth, in the rough cut she gave him tongue, and because Ford hadn't agreed to do SW3 it is Luke and Leia who are left at the end of ESB with their arms around each other (wonder how that would have played out in SW3?). Plus, Gary Kurtz has said that Leia was never the sister and that the sister was another Jedi character. Lucas says this during the ESB story conferences of 1977 and he has her character mentioned in his 1978 ESB second draft (and no, it's not Leia).

Plus, it's just obvious from watching the films.

So, conclusively, it was an idea implemented into the storyline post-ESB. There was a sister character in ESB, starting in the story conferences and ending up in the final film--the "Other"--but it was a character distinctly separate from Leia. He combined her with Leia in SW3 because he no longer wanted to do the sequel trilogy in which she would/could have featured.