CO said:

My only beef with the OT is that Luke/Leia should have never been siblings. That is when Lucas let the 'I am your father' twist go to his head and thought he could outsmart the fans with some new twist every movie after that. The Luke/Leia sibling plot point made the universe too small, and sadly 'the other' could have played a huge part in the upcoming ST.

That was the original plan, as you may have heard, but since George decided not to make the ST he wrapped things up in ROTJ. That's why it seems shoehorned in and there wasn't enough build-up to it.