Obi-Wan: "That boy is our last hope."
Yoda: "No, there is another."
Obi-Wan: "Yes, but can Leia handle the challenge? Luke's already been trained in the Jedi ways. She's never even been near a lightsaber."
Yoda: "Trust in the force, we must."
If I were in Obi-Wan's position, I'd think the same thing too. I'd put all my money on Luke saving the day. As cool as Leia is, she's much less of a hope than Luke is. Obi-Wan had a special bond with Luke. He's got all his faith riding in him. He doesn't doubt Leia's power in the force, but for her to bring down Vader? Hell no. Not in her condition. Yoda's comment was more like: "No, we have a spare."
EDIT: As I recall Luke did have a twin sister in the original huge ass script that had to be cut down for Star Wars. The line of "There is another" was put in the film to add tension for the audience but I think that Lucas knew where he was going with that (god, I can't believe that I just said that). It wasn't just a line that "sounded cool." There was more coming. Planned or not, the Luke/Leia thing works a little too well to be an accedent. Just like Vader's revelation, the brother/sister one was just as good.