Why have Owen Anakin's stepbrother?

If you really want to start from scratch have him Ben's brother.

Ben is already a proven fibber and it made sense in the ROTJ novelisation for Obi-Wan to place Luke with his brother than Anakin's.

All the resentment about damn fool idealistic crusades could be Obi-Wan talking about his relationship with his own sibling and Anakin could have been this Han style great pilot that Owen and Beru used to drink with down the Cantina when he was a hot headed slacker kid (the type of boy he keeps telling Luke off for being). Anakin could have been type of cool brother Owen always wished he had instead of some weirdo hippy mystic ninja.

Owen could be just as mad at Ben for taking away and corrupting his idealised brother as well as not being it himself.

That adds depth to four characters in one swoop.