Hereditary Diseases, in the Vampire Hunter D mythos, are a thing of the past; their names are no longer even remembered. As a result of the Nobility's genetic-engineering expertise, mental defects by inbreeding no longer exist.
A guaranteed labor force is the greatest concern on the Frontier, so relations between a mother and her son, or father and his daughter, are permitted. The exact details aren't considered important, so long as they are fruitful and multiply. However, if it is purely in the pursuit of pleasure, such as when Lina Sween or Leila's family members (or adoptive family members) attacked them, it is not condoned, and against the law.