Whatever path they took to eventually land on the most famous twist in cinematic history, the idea was not original to Lucas or Kurtz. It almost certainly was inspired by the same twist in DUNE, the novel, where Paul Atreides finds out he is the grandson of the primary villain, Vladimir Harkonnen. And so many other aspects of Star Wars seem lifted or inspired by Dune, I think it’s fairly obvious at this point, if Lucas was wrestling with anything about Vader’s backstory, it was how much ‘on the nose’ he could be without looking like he actually ripped it off directly from Dune.