The last king of the Festillic dynasty, Imre I, has an incestuous relationship with his sister Ariella, which produces the line of Festillic pretenders who for generations proceed to plague the Haldane dynasty.
Miro reached the same conclusion through rational means, reasoning that only a large community can afford to have people break religious taboos.
Unfortunately, Moll's mother is also.
They do commit incest towards the end of the book.