Hahaha! I finally figured out why Obi Wan tells Yoda, "That boy is our last hope," in The Empire Strikes Back, despite the fact that he's known all along that Leia is Luke's twin sister. It's because she is a politician -- a senator --and he hates politicians. He can't conceive of teaching her the ways of the Force and turning her into a Jedi, because in his mind, she's already halfway to being a Sith.
Obi Wan's loathing of politicians had struck me as interesting but a bit strange ever since it was introduced in AOTC. Now I realize that it perfectly explains why his line to Yoda was, "That boy is our last hope," and not, "Well, he's a goner. I guess we'd better get to work on Leia."