The Hapsburgs, a Spanish noble house that held the Spanish throne, made a practice of the inverse of this with uncle-niece marriages to keep power in the house. This was the closest marriage the church would allow. The generations of inbreeding produced the distinct “Hapsburg jaw”. Eventually that led to the poor, ugly, absurdly inbred Charles II of Spain. And remember, those portraits are official portraits that paint him in the best possible light. He died without an heir, ending the Hapsburg monarchy.