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
Nailing canon at this point in time is tricky. Every other week something would be canonised that decanonised something. Your version of events was also true at some point. Entire books have been written about the development process of the original trilogy