It depends on when you're asking the question. When this was filmed, for instance, Luke had a sister (regular, not twin) on the other side of the galaxy, just in case something happened to the real-life Mark Hamill. Once RoTJ rolled around, they needed to close off this plot thread so they reconnected the sister to be Leia. From RoTJ onwards, there's not much wriggle room on who it could be. The 'Leia is your sister' scene in RoTJ is there to remove all doubt
I thought Luke and Nellith were meant to team up to fight Vader? And that the original was meant to span over about 8 episodes (ROTJ was meant to be just about the rescue of Han) but then Lucas went through the divorce and just wanted to wrap the series up, so he retconned Leia to be the sister.
I wish we could have had the "Luke searches for and trains his sister" arc.