There’s a lot of contradictory claims about when exactly Lucas decided Vader was to become Luke’s father. The most obvious hypothesis would be some time after Leigh Brackett was commissioned to write the script, but before the Fourth Draft was completed. The Fourth Draft contains the mysterious words “(INSERT B - DIALOGUE ADDED HERE)” where Vader’s infamous line is supposed to go (presumably this was done to keep the shocking twist from leaking).
But I’ve also read the claim that Lucas actually came up with the plot twist before he commissioned Leigh Brackett, but just didn’t tell her about it. Of course, that claim seems like typical Lucas revisionist nonsense, but it would help explain why the Emperor is now an evil Force-using sorcerer in Brackett’s draft, instead of the figurehead politician. Of course, if this claim is true, Brackett’s script would need to have been changed significantly, since Luke’s father actually appears as a force ghost on Dagobah to give Luke some Jedi wisdom. (Plus if I were Brackett I would be pissed that Lucas essentially wasted my time by withholding this vital story information).
In Secret History of Star Wars, Michael Kaminski hypothesizes that Lucas came up with the idea to make Vader into Luke’s father after Brackett completed the first draft (and sadly died). Kaminski speculates that Lucas’ thought process leading up to this decision was as follows: Lucas read Brackett’s draft and something bothered him about the scenes where Luke’s father appears as a force ghost. Lucas realized that Luke’s father (who was a typical wise old Jedi character) was actually a completely redundant character, because he’s basically the exact same character as Ben Kenobi (who is also a Force Ghost at this point). But once the concept of force ghosts was introduced, it would follow logically that Luke’s father should begin appearing to Luke, since Kenobi always does. So now Lucas was stuck with duplicate “wise old Jedi” characters. If Luke’s father started appearing to Luke, there really would be no reason for Kenobi to have any further role in the story. So eventually, while struggling to find a solution to this problem, Lucas stumbled upon the greatest movie twist in history.
This hypothesis seems believable to me. It’s purely a guess, but it makes sense. But on the other hand, there is some evidence that Lucas actually decided that Vader was Luke’s father even before Leigh Brackett wrote the first draft. Kaminski points out how ridiculous this claim seems, because it means Lucas purposely withheld vital story information from Brackett, even while paying her to write the script (presumably out of paranoia of leaking the twist). And yeah, this does seem kind of ridiculous - but maybe it’s more like Lucas was just indecisive about the idea. Apparently it might actually be true, at least according to JW Rinzler, who claims to have many of Lucas’ handwritten preliminary notes for Empire Strikes Back, seemingly pre-dating Brackett’s draft. One of these note pages has the words “Father changes into Darth Vader”. But unfortunately Rinzler doesn’t provide a photograph of this (but provides many photographs of other notes), and Rinzler also admits that none of these notes are dated so it’s difficult to construct a precise timeline of Lucas’ thought process.
Anyway, if true, it would help explain why the Emperor turns into a Force-using evil sorcerer in Leigh Brackett’s script. Because if Lucas had already planned on making Vader into a conflicted character, there would need to be some new “ultimate bad guy” character to take his place.