Inbreeding is far less problematic to the gene pool that religions would have you think.

Inbreeding can result in a birth defect anywhere from 1 to 100 generations on. Birth defects happen in non-inbred populations as well, so not sure how dangerous that "it could happen the first time" is.

I remember in an anthropology class that there have been multiple points in human history where the species was severely culled. At one point, there were only 600 living humans. We are all related and inbred from those 600.