Well "Darth" was never used as a title until Episode I where Darth Maul and Darth Sidious popped up and it was suddently a title of the Sith.
But there's a lot of stuff like that that got changed when Lucas did the prequals and even ESB and RotJ. Darth Vader wasn't Luke's father until Lucas decided he was when he did ESB. Luke and Leia weren't brother and sister until Lucas decided they were when he wrote RotJ (either that or he really likes to watch brothers and sisters kiss each other), and even Uncle Owen wasn't Anakin's brother until Episode II, he was originally suposed to be Obi-Wan's brother.