Luke and Leia were never brother and sister. The other that Yoda refers to IS Luke's twin sister but you were never going to find out who she was until the next trilogy. After ROTJ Luke was going to search for his lost sister. When George was writing the climax of the duel between Vader and Luke he couldn't figure out what vader could say to Luke that would make him go nuts. So he decided that it should be Vader threatening to turn his sister. But, as the sister hadn't been introduced, he turned Leia into the sister. So much for the BS that he had everything planned out since he first started writing Star Wars.