I posted my counter argument earlier, but I'll do it here as well.
From the beginning of Star Wars, Vader always had a close connection to Luke's dad. They were friends, co-students of Obi-Wan, and Vader killed Luke's dad. There's already a pretty tight link between Luke and Vader. Making him his father, in my trying to be humble opinion, did not shrink the universe. Or, at least, did not shrink it by much.
Luke and Leia, on the other hand...