Here's the theory: Obviously, in ROTJ, Yoda was referring to Leia when he said "There is another Skywalker." And Obi-Wan confirmed this to Luke. However, could the line in ESB have been referring to somebody else? "That boy is our last hope." "No, there is another." Lucas has stated (I don't have the quote handy) that his original idea was that this "other" referred to someone on the other side of the galaxy, perhaps a lost sibling of Luke, who would appear in Episodes VII-IX. Now, there won't be another sibling of Luke, but it could refer to someone else who is powerful and important. Why would Yoda have said to Obi-Wan something that Obi-Wan already knew perfectly well--that Leia was out there? I think we forget that Luke himself didn't hear the line in ESB because he was flying away in his X-Wing. Luke only heard the line in ROTJ, which was different, "There is another Skywalker." The immediate answer is, of course, that George didn't have the Luke/Leia thing figured out yet while ESB was in production. This leaves the door open for the "Other" from Yoda's line in ESB to be someone else. George always wanted this person to be in the next trilogy. He thought he closed the door with ROTJ, but it's not airtight. Disney/JJ/Kasdan, etc. could exploit this to bring in this other character. This would explain the ESB line and still be accurate, you know, "from a certain point of view."