A long time ago in a galaxy far, far away, brother and sister fell in love and ruled the galaxy together ... almost.
Before you even start twitching, let me assure you of a couple of things: No, it would not have been canonical, and no, Luke and Leia would not have known that they were brother and sister, though that doesn't make it any less gross for us. This could have happened in an officially licensed Star Wars comic book published by Dark Horse Comics, but it never quite made it into production.
In the early 2000s, Dark Horse crafted a series of comic stories called Star Wars: Infinities. The premise of these tales was to retell each of the three original trilogy films by changing one crucial piece of the plot that would then create a domino effect on the rest of the story. They're basically "Elseworlds" stories for Star Wars, out-of-continuity musings with the classic characters and settings, and as Dark Horse was developing the project, they offered legendary comics writer Peter David the chance to script the first one, a retelling of A New Hope. David began crafting his idea with the understanding that he could basically do whatever he wanted, but Lucasfilm ended up rejecting his take because it was "too dark."
"I gotta say, I was furious when Lucasfilm kicked it back," David wrote on his website. "The whole reason I’d taken on the assignment was because I’d been told that Lucasfilm had effectively given us carte blanche. But then, after I drafted the outline, they turned around and said that the story had to end with Luke, Leia and Han triumphing over evil. In other words, it had to have the same exact ending as Star Wars. It’s a crying shame. The cover image to issue #2, with a close shot of Princess Leia in the Darth Maul face make-up glaring out at the reader, would’ve been killer."
Obviously, a story featuring a brother and sister becoming unknowing lovers and uniting to rule the galaxy is a bit dark for Lucasfilm standards, but how would David have arrived at that point? What crucial moment would he have altered that would have eventually led to that pairing? It all starts when Uncle Owen doesn't buy R2-D2.
"The concept was that you take one point in the film, and deviate from it, and see what happens," David said. "I actually ended up keying off my ‘Skippy the Jedi Droid’ story. I didn’t use Skippy in it per se, but I went to that moment when the droid that Uncle Owen was originally going to go with blows, and I had it not blow up. That simple. The robot mangaged to hold itself together long enough for the Jawas to make tracks, and then it blows up. But that sent everything off in a different direction. Because if that droid doesn’t blow up at that particular moment, they don’t acquire R2-D2. Without R2-D2, the story doesn’t go the way it’s supposed to go… the story ended with Leia, now the ruler of the Empire, a Sith Lord and her apprentice and lover, Luke Skywalker. They didn’t know!"
In case you were wondering, the New Hope retelling that eventually made it into Star Wars: Infinities, written by Chris Warner and drawn by Drew Johnson, imagined what would have happened if Luke's torpedoes malfunctioned at the moment he was supposed to blow up the Death Star. Luke and Han manage to get away, but Leia is captured and taken to Coruscant to be trained by the Emperor. Luke eventually has to duel her before revealing their kinship and saving her life, and that's only part of a much larger story. That's a neat idea, but whatever David would've done to get us from wrong droid purchase to Sith Leia and her brother-husband would surely have been worth a read.