In general, I see what you're accomplishing. I guess, to me, a twin is a twin, i.e., born of the same mother. My problem is that you're saying Obi-Wan is lying that Anakin came from Tatooine (I don't know why he would lie about something like that---essentially, 'what's the point?'), but there you have Owen is obviously a long-time resident of Tatooine and relative of Anakin---and he KNEW Anakin. That is, everything he says to Luke is as if he had witnessed something that Anakin did to make him hope that Luke never becomes like his father.
I think the way to diminish Obi-Wan's "lies" and set things straight and make a more acceptable storyline is to do some minor dialogue alterations to ROTJ, and then we can set up a prequels that would be considerably more satisfying.