Star Wars has been constantly retconning itself, from the beginning.

The first film was not really produced as "Episode IV", it was "Star Wars", a standalone film. It was a movie about a farmer orphan who goes on a swashbuckling space adventure with laser swords and space wizards. The good guys are unambiguously good, the bad guys are just bad guys. Everything is pretty much just as it seems, no secretly alive people, no secretly related people. Lucas may have had nebulous plans/hopes for follow ons, but they weren't baked and the overall concept is standalone.

Then ESB came along and retconned the Skywalker family, and produced cliffhangers knowing there'd be a third film. However, I'm pretty certain that "there is another Skywalker" didn't specifically have Leia in mind at the time, mainly because of how it's handled in the follow up.

Then ROTJ came along, and that little tease about 'there is another Skywalker?' just a kind of casual "oh yeah, that's Leia, and she's your sister, and we are going to do absolutely nothing serious with that, just consider the matter closed even though they were clearly setting up for... something with that".

A lot of things in the franchise have this feel. Like "Rei's provenance is mysterious and significant" swinging in the next film to "the parents are nobody, parents don't matter" and then swinging again in the last of that set of three to "just kidding, her provenance is very significant".