However, how is it to be so certain that in saying there is another that this is what it refers to. Maybe it wasn't Vaders demise they sought but his salvation. Admittedly your theory is probably correct and as already obviously highlighted it is most likely Leia on the grounds that GL seems to like linkage between the characters and she had just also ben declared as Lukes sister. Its just nice to think about it really. Theorising is always subjective even when relating to an objective factor. As such in the end of the day it just helps pass time.
More or less I was saying that in general I agree that it is probably another potential Jedi who could take Vader down. I also see it likely to be family member and as such probably Leia. What I was saying was however that it could equally be taken to mean - there is another...way. Or it may been another who they could rely on to do the job who wasn't jedi. This I know however is highly unlikely.