If the issues with interbreeding was such an issue we would have so many more problems in the Middle East and such because cousin marriages over there aren't that unusual (which as an Arab living in the USA was weird until i read it's like a 2% or something increase in birth defects for cousin marriages) and mostly a result of the arranged marriages stuff.     It's almost always when you have siblings or parent-child and that's more of a risk of recessive traits being amplified. It's the successive and repeated incest that usually is what causes major problems. Cousin marriages only increase the chances of birth defects from 2%-3% to 6%-7%     The focus on birth defects usually are mislead and unfortunately changes the discussion from "these marriages are bad because of power dynamic" to "its bad to the child's health" which tends to ignore basic genetics and biology and weakens the argument.