on why Obi-Wan Kenobi doesn't seem to consider Leia a 'Hope' (follow link for more details). His conclusions:
What do we know for sure? Obi-Wan and Yoda's opinion of Leia's status as a "hope" differed. This difference was one of opinion, not sexism or a lack of knowledge.
There are plenty of possible reasons Obi-Wan may have favored Luke... his familiarity with Luke and unfamiliarity with Leia creating a bias, Luke's level of training vs. Leia's, evidence of differing gifts as the twins developed, secret midichlorian tests, or maybe just a difference in what was being hoped for as discussed above. There simply isn't enough evidence to know for sure.
Since there's no stretch of logic (if I do say so myself) for this line to make sense, this is no plot hole.
Personally, I'd like to propose another reason, if I may: Obi-Wan, knowing the intensity of Luke's feelings and the strength of his connection to Leia, feared that if Luke had any knowledge of his blood relationship to Leia it would pose a threat to Leia. In the Emperor's throne room battle scene at the end of ROTJ, this exact situation plays out - Vader picks through Luke's mind and finds out about Leia, and this drives Luke out of hiding and into an attack on Vader (likely motivated by Dark Side emotions). Thus, I think Obi-Wan's thinking was, "If he doesn't know she's his sister, then she will be safe."
However, Yoda had a different plan. I think Ghent's theory on this point is quite valid - that Yoda was thinking longer-term and attempting to ensure the continuation of the Jedi by making sure that Luke knew that he could (and should) train Leia in the Jedi arts. Perhaps he felt that Luke training Leia was important enough to risk revealing her existence to Vader and the Emperor.
In any event, it brings out yet again the mild conflict of style and foresight/insight between Kenobi and Yoda that was (if my memory serves) brought up in Episode I. (What I'm thinking of could've been between Kenobi and Qui-Gon in E-I, or Kenobi/Yoda in E-II...it's been awhile since I've seen either.) In the end everything works out, but it brings a bit of chaos to Luke's world, with various sides of the truth being thrown at him depending on who he talks to.
<shrug> My ten cents says, "No plot hole either." :-)
(Hat tip: Slashdot)