The "there is another" line was Lucas' wiggle room if something happened to Mark Hamill between films, (he already had a nasty car accident) much as Han getting frozen gave everyone an out if Harrison didn't want to come back. The old "Making of a Saga" documentary strongly implies they didn't come up with Leia being Luke's sister until they were on the set of Jedi, trying to come up with something for Vader to say to provoke Luke into fighting him.