Where did the mistaken idea of uncle Owen being Obi-Wan's brother originate?

Question

In the long interim between Return of the Jedi and Attack of the Clones, I saw a lot of claims that Luke's uncle Owen was actual Ben Kenobi's brother.

I figured from the first time I hear this that the idea arose from somebody misinterpreting an ambiguous pronoun reference (as in: it was written down somewhere—amidst mentions of Obi-Wan and Anakin—that Owen was "his" brother, and somebody inferred the wrong referent). It seemed obviously incorrect to me, given Obi-Wan's attitude toward Owen Lars, but some other Star Wars fans I talked to insisted on it—at least up until the issue was conclusively cleared up in Episode II.

But where exactly did this misapprehension get started? Specifically, is there an identifiable source (such as the novelization of one of the original films) that was misinterpreted? And where did the erroneous claim first appear in extended universe materials?

Answer 1

It was canon (for a while), not a mistake

In the 1983 ROTJ novellization, Obi-Wan tells Luke — after Yoda's death, when he's telling him about how Anakin is more machine now than man — that he took Leia to live on Alderaan and Luke to Tatooine:

"to live with my brother, Owen."

Based on this expanded universe information not being contradicted, as was LFL’s canon policy for many years, this information was then propagated throughout all of the official reference books that were published. For example, this also appeared in Star Wars: Essential Guide to Characters based on the expanded information from the novelization.

Answer 2

This was introduced at least as early as the second draft of the script for Return of the Jedi. According to Star Wars: The Annotated Screenplays (1997) (emphasis added):

It was decided during story meetings that Uncle Owen was in fact Ben's brother. Owen always resented Ben for imposing Luke on them, and now Ben is taking the blame for what is happening and is feeling guilty.

[...]

In the second draft [...] Ben reveals to Luke that he has a twin sister and that they were separated; Luke was sent to stay with Ben's brother, Owen, on Tatooine, while his sister and mother were sent to the protection of friends in a distant system. The mother died shortly thereafter, and Luke's sister was adopted by Ben's friends, the governor of Alderaan and his wife.

(Star Wars: The Annotated Screenplays, p 269-270)