Exactly. ROTJ is episodes 7-9. They had to cram everything in there to get it done, and Leia just magically became his sister. I think it works brilliantly. Hell, even the names resonate with what twins often are stuck with.
Plus, the idea that the two of them were separated at birth and yet spend the next 3 movies together is wonderful. Intentional or not, I wouldn't have it any other way.