A proposition submitted for consideration:

George Lucas said in 2007 (when interviewed for The Making of Star Wars), “It was at that moment [while writing the third-draft script in 1975] that I came up with the idea that Luke and the princess are twins. I simply divided the character in two.”

For once he wasn’t exaggerating or stretching the truth. He was simply being honest - after all, the 1975 second-draft script of The Star Wars featured Luke Starkiller rescuing his brother Deak from an Imperial prison, with Deak essentially playing the Princess Leia role. And Ralph McQuarrie’s sketches of Luke and Leia from 1975 show the two characters as both blonde with identical bowl cuts.

So – although the idea was likely abandoned during the production of ESB, and then hastily resurrected for ROTJ – I think GL might actually be telling the truth when he suggests that he considered having Luke and Leia be siblings (well, half-siblings at least) from the very beginning.

With all the incest that implies.