It’s not just the ‘creepy’ aspect of the two of them being brother and sister that I hate, in fact the main reason why I don’t really love ROTJ as much as the other three good Star Wars films is because of how convenient the whole movie is.
Luke needs to return to Dagobah to finish his training;
“Hmm, just make Yoda die. That way he won’t need to finish his training, and we won’t have to address the fact that Luke was supposed to be making a reckless decision in facing Vader without proper training.”
There is another hope for the galaxy that is heavily implied to be Luke’s sibling;
“Meh, just make Leia Luke’s sister. That way we don’t have to introduce any more characters AND we can dissolve the love triangle between the two of them and Han.”
I don’t know, it just seems a bit lazy to me.