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.
Yes, it’s true that many of your complaints are the result of the late decision to end the trilogy with Jedi when 9-12 films were originally planned. That being said, I think it wraps things up naturally when you take that into account, about as smoothly as it possibly could in fact.
Plus, you can explain that stuff. Luke needed to be the last Jedi, he needed to face Vader on his own. So Yoda had to die for that to happen. Not to mention it was addressed that Luke made a reckless decision to face Vader in the last movie - clearly he learned from his mistakes. He constructed his own lightsaber, and from the start of Jedi it is clear he is much more wise and careful. I never thought Luke needed that much literal training, as in like a certain number of hours of lifting rocks, to become a Jedi. I always interpreted it as more of a getting in sync with the force kind of thing that comes with inner understanding, like a spiritual thing. Of course the prequels undid that, but let’s just forget about that…
And again, I point out that Leia does seem to be implied to be the one Yoda was speaking of in Empire when she senses Luke. That tells us both that she is force sensitive and that she has a special connection to Luke. If I was walking out of a theater in 1980 speculating who Yoda was talking about, she would have been my guess based on that. Also it’s for the best that a character we already knew was used rather than another character shoehorned in for the last movie. People would just complain about that too.