If we are talking OT continuity then "another" refers to Leia...

...but if we're talking PT continuity I have no bl**dy idea who Ben is referring to in that line. It can't be Leia because why would Yoda have to tell Ben that, they were both at the birth. Your guess is as good as mine???

Best just ignore the PT insanity.

Having said all that "another" could refer to an as yet unseen character in EpVII. I'm not sure if that would be "Oooooo goosebumps cool back refrence" or "Arrgghh stop messing with OT continuity"?