What’s wild is that this particular idea - unlike the ‘Anakin dead but Vader is Luke’s real dad’ idea or the ‘Vader as clone of Luke’s dad’ ideas - was on-record as actually an idea considered by George Lucas at one time (while ANH was being developed/written, no less). And also that this idea - along with the Vader-is-a-clone-of-Dad idea - was part of a magazine article speculation from a 1981 issue of Fangora magazine. Though I suspect it may have had it’s origins as a genuine leak from LFL - possibly from before Gary Kurtz had departed from the company - rather than just a lucky guess or sheer coincidence.

As far as how things played out in Empire and Jedi:
I think more and more it’s looking to me that with the second draft of ESB (written solely by George) one could say that he did by then have the basic story/plot for Jedi (ROTJ) written - at least as far as the Vader-is-Anakin twist plus maybe even Leia as the sister/Jedi Other hope. I think by then he had not only dropped the ‘Neilith Skwalker’ idea* as Luke’s sister/the Other hope, but he had also dropped his plan** that he (together with Kurtz) had of the saga’s story extending to nine episodes (let alone twelve), and had sort of went back to his end-of-1975 proposal of the saga just going to be three movies only (not counting the prequel back-story episodes).

*see the first draft of ESB by Leigh Brackett
** where the Emperor presumably wouldn’t appear in-person in the story until Episode Nine