For a number of years, fans have believed that Owen Lars and Obi-Wan Kenobi were brothers. And there was good reason for that. The novelization of Return of the Jedi (1983) had the following line:

Ben continued his narrative. "When your father left, he did not know your mother was pregnant. Your mother and I knew he would find out eventually, but we wanted to keep you both safe as possible, for as long as possible. So I took you to live with my brother Owen, on Tatooine...and your mother took Leia to live as the daughter of Senator Organa, on Alderaan."

Then there's the following line from the entry for Owen Lars in "A Guide to the Star Wars Universe" by Raymond L. Velasco (1984):

OWEN LARS - a moisture farmer from the planet Tatooine whom Luke Skywalker believed to be his natural uncle. In fact, Owen was the brother of another protector of Luke: Ben Kenobi.

It also appears again in "A Guide to the Star Wars Universe - Second Edition" by Bill Slavicsek (1994).

Luke believed that Owen was his natural uncle, but in fact the man was Ben Kenobi's brother.

Then, as recently as August 1999, Jedi Apprentice #3 - The Hidden Past by Jude Watson had the following quote during a scene where Obi-Wan attempts to fight a memory wipe. He tries to remember things from his past in order to fight it:

Twin metal rods were scrwed against his temples. The electro-pulsers.

The stone glowed against his heart.

A visit home. His mother. Softness and light. His father. A laugh, full bodied, joined by his mother's, just as full, just as rich. His brother, sharing a piece of fruit with him. The explosion of sweet juice in his mouth. Soft grasses underneath his bare feet.

The droid activated the memory wipe while the guards watched. A strange sensation began in his temples and moved inward. Not pain, not quite...

Owen. His brother's name was Owen.

So as you can see, the notion that Ben and Owen were brothers is not some fan made thing. However, it's looking more and more like all of this is going to be disregarded in Episode II (much like the expanded universe history of Boba Fett as discussed the other day). Lucasfilm has always said that the films were the only true canon, and that keeps things wide open for Lucas to work with. He's not tied down by the countless books, comics, and games when making the prequels. In fact, Lucas has shown that he's willing to revise the history written by the films too. After all, we all thought Han shot first until the Special Editions, right?

Guess we need to alter that family tree again, eh?

UPDATED!! - The ever vigil Star Wars fans out there have found a few more references to Owen and Obi-Wan being brothers. The first is from Morgan, writer of the Jawa Force fan comics:

Here's a reference to Uncle owen and Ben on the old Star Wars Screensaver.

Then, probably more importantly, is this from ObiWanJane:

Your "Regarding Owen Lars and Ben Kenobi" analysis is missing one vital element: an exerpt of ROTJ story notes from the OT Annotated Screenplays. Page 269, fourth paragraph:

"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."

Why is this more important? Well, the book is from 1997 and the author, Laurent Bouzereau, had access to not only the film notes and early drafts, but he talked with both Lawrence Kasdan and George Lucas about the writing of ROTJ. That indicates that the first reference to this relationship in the ROTJ novelization was not a simple line thrown in by author James Kahn. It was taken from notes by Lucas. But it is still fair game for ignoring as we mentioned above.

UPDATE 2!! - Adam Garcia mentioned that The Essential Guide to Characters (1995) by Andy Mangels also has a reference to the relationship under the Owen Lars entry:

Owen Lars was never as flashy as his brother, Obi-Wan Kenobi, one fo the strongest Jedi Knights, nor as daring as Kenobi's friend, Anakin Skywalker.

Steve also found mention of the brother relationship on the first set of Star Wars CCG cards. You can see Uncle Owen's card here. Thanks for the additions!