Yeah, I liked Casey's interpretation from way back on why Obi-Wan said Luke was their last hope. (And we've known since ROTJ that OB1 always knew about Leia.)
Casey said that OB1 knew Leia was too much of a bitch to train her as a Jedi. She'd have become Sith for sure.
Maybe a few years of getting laid by Han would make it safe, but at the time OB1 knew her she was too much of a risk. Perhaps Yoda foresaw that a rogue smuggler would put her in her place.

Yoda still contradicts himeslf thanks to the quick-fix of Leia being Luke's sister thing though. He says there's another hope, but at the same time he wants Luke to let her be killed. Then again, he only brings up the "another" thing AFTER Luke makes his choice, so I guess Lucas has grounds to defend that.
The Anakin as "another" interpretation works ok too though since it's OB1 encouraging Luke to destroy Vader. Yoda doesn't really tell Luke to kill him in ROTJ. So, it's possible OB1 and Yoda have different opinions on Vader. Then again OB1 says "the other he spoke of is your twin sister" speech in Jedi (though this time after the "another Sky-wal-ker" bit. I would take that as Lucas' implication that the other Skywalker and another hope are one and the same, Luke's sister.
At the time of filming ESB, I feel confident the plan wasn't to have Vader destroy the Emperor since he wasn't to be taken out of the equation until Ep. 9 I believe. So, the sister was the intention at the time, and I'd wager we're supposed to think Yoda and OB1 both consider Vader is lost for good. Only Luke believes in his father's goodness (perhaps inherited from his mother), and it's only at the end of ROTJ that it's revealed the prophecy was, in fact, correct.