^I like that. If ambiguity were kept throughout the whole movie as to who was Luke's sister, you could even have Vader/Luke say in Luke's voice, "I am your sister," and then die. This could even be extended to ESB where Vader might say, "Obi-wan never told you what happened to your twin sister...I am your sister!" And statements that "Sister...I am your twin sister," could send Luke into the rage that ultimately leads to defeating Vader. Heck, it could even go back to ANH with Luke asking, "He betrayed and murdered your sister," and Luke saying, "But I have no sister." The new "sister" plot could be the ridiculous thread that ties the whole trilogy together. To tie up other lines that wouldn't match the plot, you could ultimately reveal that someone else is his father, like Obi-wan himself or the Emperor.