From this 1977 TV commercial for Star Wars: "Luke Skywalker and Princess Leia in danger. In love. In Star Wars. No legendary adventure of the past could be as exciting as this romance of the future."
Here's Hammill's response from a few years ago when someone asked him about the revelation in Return of the Jedi that Luke and Leia are brother and sister, yet had smooched in the previous film The Empire Strikes Back: