Luke: I can't do it, Ben.
Obi-Wan: You cannot escape your destiny. You must face Darth Vader again.
Luke: I can't kill my own father.
Obi-Wan: Then the Emperor has already won. You were our only hope.
Luke: Yoda spoke of another.
Obi-Wan: The other he spoke of is your twin sister.
Luke: But I HAVE no sister.
Obi-Wan: Hmm. To protect you both from the Emperor, you were hidden from your father when you were born. The Emperor knew, as I did, if Anakin were to have any offspring, they would be a threat to him. That is the reason why your sister remains safely anonymous.
Luke: Leia! Leia's my sister.
Obi-Wan: Your insight serves you well. Bury your feelings deep down, Luke. They do you credit, but they could be made to serve the Emperor.