Inbreeding. By definition it is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically, in contrast to outbreeding, which refers to mating unrelated individuals…
I have written about this before. It does not refer to marriage between brothers and sisters—that seems to be a no-no everywhere.
The marriage partners are more likely to be cousins. The rates of cousin marriage in Muslim countries are staggeringly high (see link).
Along with all genetic disadvantages it also is the basis of tribes and clans—the enemy of democracy everywhere.
Had the Church not banned the practise centuries ago in Europe it is likely democracy would never have emerged at all. How long will it survive at this rate?