The tread title has had me confused. I'm guessing it should say:
'What did Yoda mean?'

This subject seems to pop up often in Star Wars discussions and I'll never know why.
In ROTJ Obi-Wan says "The other he (Yoda) spoke of is your twin sister."
So, that's that.

What it originally meant when the Empire script was written is up to debate, but as far as the trilogy goes, he was talking about Leia.

Ryan McAvoy said:

"another" could refer to an as yet unseen character in EpVII.

Except that Obi-Wan has already let us know what Yoda was talking about.
=p