Player characters might be able to win a fight with the Big Boss villain, but they should never be able to win an argument with them. Enforce this rule for all it's worth.
Darth Vader: ...oooo...
Luke: You can't kill me now - you're my father.
Darth Vader: No. I know that's not true. That really is impossible.
Luke: Oh come on, I figured it out. All very clever.
Luke: Princess is your daughter, adopted at birth by Bail Organa and Brea Moon-Flower. And I'm her twin brother, placed in the care of Owen and Beru Lars on Tatooine.
Luke: Ergo, you are Anakin, my father.
Luke: You killed Padmé, the woman you loved. My mother!
Darth Vader: No.
Luke: You are my father!
Darth Vader: No, Luke. I am your mother.