gobalicious said:

He should have just said, "Leia is your sister." On your deathbed is a good time to be efficient with words. Besides, it's kind of pointless drama anyway, since the question of who the other Skywalker is gets resolved almost immediately when Luke talks to Obi-Wan.

If Yoda had not mentioned that there is another Skywalker, Luke wouldn't have asked Obi-Wan about it, and Obi-Wan would not have said anything about it to Luke, and then Luke would not have realized that Leia was his sister.

You call that pointless?