They weren’t related when he wrote the movie. No mythology, no influences, no anything else. It was a farm boy and a princess in the only thing Lucas had written. There was no Original Vision or multi-film Saga
I meant earlier drafts with Annikin Starkiller and Deak Starkiller. The story evolved away from siblings to arbitrary come together in Return of the Jedi with Luke and Leia becoming twins. I got my information crossed on the twins part.
There’s a lot of mythology that has Luke and Leia like relationships at the same time. So I do think there’s some merit as to why he’d merge the two than simply wrapping up the loose end of “Another”. Although arguably that was part of it because he was burnt out by Star Wars and wasn’t going to do Sequels at the time.