How we cite our quotes: All quotations are from Star Wars: Return of the Jedi.
Quote #4
LEIA: Luke, what's wrong?
LUKE: Leia, do you remember your mother? Your real mother?
LEIA: Just a little bit. She died when I was very young.
LUKE: What do you remember?
LEIA: Just...images really. Feelings.
LUKE: Tell me.
LEIA: She was very beautiful. Kind, but sad. Why are you asking me this?
LUKE: I have no memory of my mother. I never knew her.
In this exchange, Luke's working his way around to telling Leia that they're brother and sister. We can't blame him for taking his sweet time. "Surprise! We're twins! You owe me, like, twenty years of Christmas gifts!" is a lot to process.
Quote #5
LEIA: Luke, tell me. What's troubling you?
LUKE: Vader's here, now, on this moon.
LEIA: How do you know?
LUKE: I felt his presence. He's come for me. He can feel when I'm near. That's why I have to go. As long as I stay, I'm endangering the group and our mission. I have to face him.
LEIA: Why?
LUKE: He's my father.
Luke and Darth Vader have kind of a Harry Potter-and-Voldemort thing going on, don't they? These two are bonded so tight that they can feel each other's presence and sense each other's thoughts and feelings—which means, had Luke grown up with Vader, it would've been virtually impossible for him to lie about a broken vase or something.
Quote #6
LUKE: If I don't make it back, you're the only hope for the Alliance.
LEIA: Luke, don't talk that way. You have a power I don't understand and could never have.
LUKE: You're wrong, Leia. You have that power, too. In time, you'll learn to use it as I have. The Force is strong in my family. My father has it. I have it. And my sister has it. Yes. It's you, Leia.
LEIA: I know. Somehow, I've always known.
We're prone to believing Leia here, even though we still wonder why, if she knew, she never spoke up. Still, if she's a Skywalker, that means the The Force is sturdy in her, so it's totally believable that she sensed that she had a brother out there somewhere.