I suppose Obi-Wan and Owen *could* be brothers. The prequels need not necessarily preclude it. Cliegg Lars moved to Tatooine some time after giving up Obi-Wan to the Jedi Order. (The difference in their last names isn't a counter-argument, because it would have been a consideration at the time of the ROTJ novelization anyway.)
Without a religious hierarchy with infallible pronouncements to interpret this for me, my computerized mind produces logical rationalizations. My mind is going. I can feel it.