Apart from DNA they have magic in their veins
Yes! Thats what hit me too incest = genome mutations -> reason? Royal families
So magic nullifies genetic illnesses, maybe
Maybe having magical capabilities IS a physical disorder lol
They're insane. So mental illness. I mean Hugo did say that killing calms his blood, maybe that's an indirect way of the author saying that "Hey so this guy can only be calmed down by letting fresh warm blood which is like how 1000x worse of how u normally calm down people with sever mental illness". Plus the late dukes were abusive and rash (Yall did u not see their dad, if he's like that, so his father maybe is like that too.) This why the original Hugo was considered a failure cuz that guy is a soft as cotton candy while this new Hugo is the perfect one (Mentaly and Physicaly perfect). Also the Taran family has this rule of killing barbarians a little, let them populate , don't destroy their core and dont let them have a leader. Cuz they use barbarians raids to hide their nasty murder frenzy.
Well there's an easy away to answer your question. This is fiction.
I have no idea if the logic works in this universe, but children born from incest relationships tends to have mental and physical disabilities... Shouldn't this be a problem? (⊙…⊙ )