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Maybe I'm mistaken but it's never mentioned that they are step-brothers that I recall.
One of the biggest plot holes I've found in starwars is Owen Skywalker relation to anikan. In the prequels he was no where to be seen, as well as anikan being introduced as an only child with no other family. Basically an orphan except for his mother. It's possible to say he's just referred to as "uncle owen" like a family friend is reffered to as an aunt or uncle. Except HIS LAST NAME IS ALSO SKYWALKER. SO WHERE DID HE COME FROM!?
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.
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)