RotJ mentions twins. I don't remember if it comes up in the Obi-Wan and Luke conversation, I'm fairly certain it does not in the Luke and Leia conversation, but I know it does in the Vader and Luke duel. When Vader pulls the information from Luke he says "Sister. So... you have a twin sister. Obi-Wan was right to hide her from me. If you won't turn, maybe she will," and Luke comes out swinging to protect Leia.

By the way, that entire quote may not be word-for-word since it's been a bit since I last watched RotJ, but I know the twin thing is right.

And Carrie Fisher is younger than Mark Hamill.