Remember this 1975 Ralph McQuarrie sketch of Luke and Leia as identical blondes?
The 1975 third draft reintroduced Princess Leia as a character, after her near total absence in the second draft… where Leia’s role in the film we know as taken by Luke’s older brother.
As awkward as the whole Leia-sister reveal in ROTJ is – mainly because during production of ESB she was very definitely not Luke’s sister – Lucas probably did consider having them be half-siblings while making the first film. And having them carry on a romantic relationship despite that.
Further circumstantial evidence:
There are strong Arthurian parallels with Ben Kenobi’s character in the 1975 third draft: he has a mechanical right hand, like the one-handed Bedivere, the knight who returned Excalibur to the Lady of the Lake after King Arthur fell to Mordred in the Battle of Camlann. Ben, as custodian of Anakin’s sword, plays a similar role, in this case after Anakin fell to Vader in the Battle of Condawn.
Besides being Bedivere to Anakin’s Arthur, Ben also plays the role of Merlin for the new generation of heroes. And Merlin failed to stop Arthur from having sex with his own half-sister Morgause, which is what led to Mordred’s birth.
Incest between half-siblings of different ages also features in the story of Richard Wagner’s Ring Cycle, where it’s blonde lookalike siblings Siegmund and Sieglinde who confess their love for one another and get it on, resulting in the birth of the hero Siegfried.
Wagner famously called his Ring Cycle “a trilogy with a prelude”. In 1975 Lucas told Alan Dean Foster he envisioned Star Wars as a trilogy of films about Luke and “the Skywalker family”, along with a single prequel movie, the same structure as Wagner’s opera cycle.
So basically Lucas was looking at and incorporating into SW a lot of ideas from stories which also feature quite prominently a brother and sister getting it on.
Not conclusive… but definitely thought-provoking.
“That Darth Vader, man. Sure does love eating Jedi.”