Obi Wann (ghost) talks to Luke after the Yoda/Luke scene.
"Yoda spoke of another".
"The other he spoke of is your twin sister".
"But I have no sister".
"To protect you both from the Emperor, you were were seperated when you were born. The Emperor knew as well as I did, that if skywalker would have any offspring, they would be a threat to him. That is why your sister remains safely anonymous".
"Leia, Leia's my sister".
"Your insight serves you well, bury your feelings deep down Luke, they do you credit but they could be made to serve the emperor".
Actually, it wasn't Leia, well, not to begin with.
When Lucas made ESB, he wanted to make Luke "expendable". Seeing as he was the only Jedi up for the job, Lucas/Kirshner realised that the audience would realise Luke WOULD always win.
If you watch the DVD's, the "other" was going to be another Jedi who survived the purge and was hiding out like Obi-Wan and Yoda. It was merely there to make the audience think Luke COULD be killed off and not have a big effect; make him expendable. It was only for dramatic purpose, and only during the making of ROTJ, did they decide to follow it up.
By ROTJ, Lucas decided he wanted to stick to his original idea of having the Skywalker Twins, and finally got his wish by having the "other" as Leia.
Hope that answers any questions.
so when Yoda says "no, there is another" did Obi-wan know he was talking about Leia?
If he did, why would he say "that boy was our last hope?" Unless he didn't believe Leia could be trained...or simply forgotten about her.
if it's just a plothole, then whatever...but if anyone has an explanation, i"d like to hear it.