They DO know about inbreeding. They aren't stupid. But, the continuity of the family outweighs the risks.
And they LOVE and cherish even their most handicapped children. No throwaway children, no abortion.
We saw that with our own eyes.
That is heart warming to hear, but ‘the continuity of the family’-wealth, land, power, prestige, etc. out weighs the welfare of their offspring.
At the very least they could submit to genetic testing and choose one cousin over the of another based upon those results.