Think of it as dramatic irony.

and if it was decided during the movie no, it was decided AFTER the movie. "He's dusted, busted and disgusted, but he's ok"

Wow, judging by some of the answers I see that some people still believe the urban myth that Lucas had it all planned from the start. Amazing. When will these dumb kids ever learn ? EVERY movie goes through constant re-writes, there is nothing bad about it : it's how movies are made. Dumb kids think we are insulting Lucas by saying he was making it up as he went along: it proves that they know nothing about movies. And that's why these stupid kids like the prequels: because they know nothing about movies.

Genetic attraction. In real life, people frequently hook up with unknown siblings because we are hardwired to go after people that look like our parents (and thus yourself and other relatives as well, if the absence of these two actually knowing their folks is an issue. Sometimes only after they've absolutely fallen in love for months or even years do they find out. It's actually quiet common (as far as bizarre shenanigans is concerned) and quite funny/heartbreaking. So it wasn't that weird. Just write it off as a really well researched bait and switch anchored in real life phenomenon. "World needs bad men. We keep the other bad men from the door"

Producer Gary Kurtz is quoted as saying: "She’s not his sister. That dropped in to wrap up everything neatly. His sister was someone else way over on the other side of the galaxy and she wasn’t going to show up until the next episode." The next episode being one they never ended up making. This article explains it pretty well: http://www.huffingtonpost.com/brian-cronin/luke-and-leia-kiss_b_8817166.html There's also the deleted scene which was meant to be just prior to Leia kissing Luke. In which Luke and Leia almost kiss but are interrupted (of course by Threepio). I couldn't watch it the whole way through the first time , because it was so awkward and weird to see such a clear romantic attraction and sexual tension from both of them. It wasn't like that in the movie. I mean, the kiss is weird enough, but at least it could be explained with Leia trying to make Han jealous. The deleted scene makes you see the kiss in a whole new light, indicating that it was less about trying to make Han jealous, than it was about getting the kiss with Luke that Threepio had cheated her of. Actually, it would have changed the whole dynamic of the movie. You'd be much more unsure after this movie about who she'd end up with, as, after all, Leia keeps kissing Luke on the lips even after Han is frozen in carbonite. I honestly think there's no way they could have made them siblings later if they had kept that scene. ----------------------- "The best fairytale is one where you believe the people" -Irvin Kershner

Thanks - that's interesting.

Check out the excellent documentary, Empire of Dreams. I think it's there that they talk about 2 things Lucas had originally drafted (Vader as Luke's father, and Leia as Luke's sister) in his original backstory notes, but then changed back and forth a number of times before and while shooting Empire. He finally decided to go forward with the original storyline of Vader being Luke's father but still had not committed himself to Leia being Luke's sister and that's why the kiss existed at all. It was somewhere between Empire and Jedi that Lucas finally decided to go with Leia as Luke's sister storyline. The thing that seems to get lost with fans all these years later is that the stories were in constant flux and development. Things went back and forth, like they do from draft to draft of any movie.

Yeah, I got that she was trying to make Han jealous, but there still would have been some awkwardness there once they realised who they were, wouldn't there? I'm just glad they didn't show the scene where Leia didn't try to make Han really, really, really jealous. 

The scene was about Leia giving Han a hard time.