Towards the end of the Empire Strikes Back when Luke is flying away in his X-Wing to face Vader. Ben's ghost says to Yoda:
That boy was our last hope.
To which Yoda replies:
No...there is another.
Referring to Leia of course.
How does Ben not know about Luke's twin sister at this point? As we saw in Episode 3 he was there during the birth of both children, and gave Leia to the Organas on Alderaan. Is there an explanation for this conversation between Yoda and Ben?
From the script of The Return of the Jedi, we have the following exchange
BEN: Then the Emperor has already won. You were our only hope.
LUKE: Yoda spoke of another.
BEN: The other he spoke of is your twin sister.
LUKE: But I have no sister.
BEN: Hmm. To protect you both from the Emperor, you were hidden from your father when you were born. The Emperor knew, as I did, if Anakin were to have any offspring, they would be a threat to him. That is the reason why your sister remains safely anonymous.
LUKE: Leia! Leia's my sister.
It is certain that Obi-Wan was aware of Leia, having given her up to Bail Organa. It is also clear that Obi-Wan doesn't believe Leia to be a hope in defeating the Emperor. This is likely because Obi-Wan believed to defeat Vader one would have to complete their training, and Luke having gotten the furthest was their only hope. Leia having never attempted the training would not have been successful, in his mind.
Originally, Luke and Leia were not supposed to be brother and sister, and Leia was clearly not Force sensitive because Vader could not detect that while torturing her, although he instantly detected Force ability in Luke while they were fighting over Death Star 1. and of course there is famous kiss.
Later, Vader is still not Luke's father and Luke got twin sister but this was not Leia. Lucas then changes the story because of financial concerns (he only get to shoot 3 movies instead of planned 9 or 6 ) . Therefore, Vader becomes Anakin and Leia is revealed to be his daughter, but apparently Kenobi does not know that until told by Yoda. Therefore, Leia is another hope or "another Skywalker" . Note that this was version at the end of original trilogy .
Finally, there is prequel trilogy. Kenobi now knows that Leia and Luke are twins, so he probably considered her not powerful enough in Force to be another hope. This explains why Vader didn't took interest in her while she was captured in the beginning of New Hope.
Also, there is one more detail to further confuse matters. In the end of Clone Wars Yoda learns from Force Priestesses. He has many visions, one of them tells him that Anakin Skywalker is the key for defeating the Sith and that he (Anakin) must be saved by all means. Another gives him his final words there is another Skywalker, which now have altogether different meaning. "Another Skywalker" is no longer Leia but instead redeemed Anakin, one that turned away from Dark Side - something that nobody considered possible, and fulfilled his destiny as Chosen One, absolute shatterpoint of Sith.
Within the novel, From a Certain Point of View, it states that Yoda thought that he would be training Leia on Dagobah, whom he perceived to be their only hope.
However, within the Rebels episode, Twin Suns, it clearly states that Kenobi believes The Chosen One and their only hope to be Luke Skywalker instead of Anakin or Leia.
Maul: Why have you come to this place? Not simply to hide. Oh...you have a purpose here. Perhaps you are protecting something? No. Protecting someone.
Maul: Tell me...is it the Chosen One?
Kenobi: He is.
This is even supported within Return of the Jedi in an exchange between him and Luke:
Luke: I can't kill my own father.
Kenobi: Then the Emperor has already won. You were our only hope.
Luke: Yoda spoke of another.
Kenobi: The other he spoke of is your twin sister.
Kenobi and Yoda had differing opinions as to who their "hope" was. Kenobi believed it to be Luke while Yoda believed it to be Leia. This explains why Kenobi and Yoda would have this exchange in Empire; Yoda was reminding Kenobi that Luke wasn't the only Skywalker left, there was still Leia:
Kenobi: That boy is our last hope.
Yoda: No. There is another.
This would also explain why Yoda would tell Luke of his sister while Kenobi never mentioned it. Yoda wanted Leia to help Luke carry the Jedi legacy after his demise:
Yoda: Luke...when gone am I...the last of the Jedi you will be. Luke..the Force runs strong in your family...pass on what you have learned...Luke..there is another...Skywalker...
However, another explanation is that the "other" Yoda was referring to in Empire was Luke's original twin sister, Nellith (who was supposed to already be a Jedi that Luke would find in the next trilogy) instead of Leia. But then Return of the Jedi rolled around and George Lucas changed Luke's sister from being a different character called Nellith and made her instead Leia Skywalker-Organa, Luke's long-lost twin sister:
However, for the purposes of this article, Luke’s sister, “the other,” as Yoda foretold, became Princess Leia as George finally decided upon once the twelve part saga became truncated into just three movies. But just who was this “other” that George originally envisioned? To make a long story short, the other, Luke’s first sister, was supposed to be a woman named “Nellith.” Nellith Skywalker was supposed to be revealed on Dagobah in Episode V, NOT by Master Yoda but by the ghost of Anakin Skywalker according to a snippet of dialogue from the first draft of The Empire Strikes Back.
Nellith was originally intended to be explored in Episodes VII through IX as Kaminski tells us:
During the story conferences with Empire Strikes Back writer Leigh Brackett in late 1977, she and Lucas also came up with some ideas for future episodes. Namely, that Luke has a twin sister. She is revealed to have been going through Jedi training at the same time, and was hidden on the other side of the galaxy by their father so that if one was killed, the other could survive. She was discussed in the first draft of Empire Strikes Back, and named Nellith in an early copy of that draft, and was set to appear in a future episode after this intro, perhaps with the two Skywalker siblings teaming up to battle their father’s killer, Darth Vader. Their actual father, Anakin, appears to Luke on Dagobah as a spirit and inducts him into the Jedi way.
George Lucas, sick and tired of producing Star Wars, turned “Nellith” into Leia...
Leia was already a nascent Force user. We see this at the end of ESB, where she feels Luke call to her through the Force. I believe Obi-Wan simply didn't know this, and was convinced that Leia was not a potential Force user, and thus unable to rebuild the Jedi.
It's likely that Yoda had been able to feel her through the Force. Obi-Wan himself says Yoda is stronger with the Force. Also, Dagobah is a nexus for the Force, and Yoda had no other distractions. By contrast, Obi-Wan was living in a much more dangerous location, forcing him to lay low. So Obi-Wan wasn't in the best position to feel someone just starting to pull at the Force. It's also possible that Leia had only recently awakened, due to her torture on the Death Star ("Her resistance to the mind probe is considerable").
I've always read Yoda's admonition to Obi-Wan as "I know something you don't", as opposed to "Don't forget about the twin"
The "other hope" was turning Vader, exactly as it finally happened...
Certainly when Yoda says in TESB "No...there is another." it does give the sense that Obi-Wan does not know about Leia being Luke's sister. And in Episode IV Obi-Wan does not give any indication at all that he knows that they are brother and sister. If Yoda thought Obi-Wan knew about her he'd say "No...his sister is just as strong". So essentially it George Lucas hadn't made them brother and sister at that point and is a contradiction with the events of Episode III.
