Yeah but Obi Wan and Owen being brothers was in one of Kasdan's drafts, Not sure if it was a Lucas idea, a Kasdan idea.

I always assumed it came out of the story conference like what originally happened to Leia's real mom. But since i have no access to the entire story conference transcript i cannot be sure 100%.

We also know that in the production of each of the original trilogy films Lucas was making things up as he went and could change things as he went along.

Though for some reason the mythology of it being a whole cohesive saga planned out from the beginning persisted until the prequels shot that belief to hell.

Unless you are someone who finds an excuse to believe otherwise, i mean if you believe the tragedy of Darth Vader stuff and the six film saga.