I'm not a fan of the sister thing, but I did like how Vader used it to goad Luke into fighting him. He started with Luke's friends, figured out he had a sister, invoked Obi-Wan's "failure" just to piss off Luke even more, then raised the possibility of turning Leia to the Dark Side. It was Vader's last truly dark moment, and I wonder how well it would have worked without the existence of Leia as sister. Sure, maybe it would have worked as Luke's love interest, but I don't believe that this particular scene doesn't work as it is.

And ROTJ brought perhaps my favorite moment of any Star Wars movie:

"Never. I'll never turn to the Dark Side. You've failed, Your Highness. I am a Jedi, like my father before me."