In Empire Leia was not Lukes sister (my opinion is based on GL not making brother and sister kiss romantically) . The story was changed during the writing of ROTJ. (GL choosing to forget about what he had previously written).

On Dagobah, when Luke left to rescue Han and Leia, Obi Wan said "That boy is our last hope." Yoda says "No, there is another."

Obi Wan was at Luke and Leias birth, so now we have a continuity error or it was not Leia Yoda was talking about on Dagobah.

If it was not Leia, who was Yoda referring to?

This could be the person who is the hero of the next trilogy or two.