What is the effective relatedness of inbreeding?
In "The Selfish Gene" Dawkins claims a relatedness factor of 1/128 to be pretty much the same as the baseline chance for the rest of the population to have the same gene, so that would be a third cousin. (Or your great great great great great grandparents, though an incestuous relationship with them is quite unlikely I think.)

I'm not sure where he got that from though.