Someone played to much Crusader Kings...
In a word, impossible. Not going to POD lands, cause it would be both hialrious and creepy regarding this challenge, Earth would become flat before sucession rules would allow the Duke to be king.
For instance : the first male offspring, if not the first female, if not the closest male relative of the former king.
Seeing how matrimonial alliances are far more common with foreign lines in order to make alliances, treaties, claims, rather than their own colonial nobility (whatever its origin, spanish or indian, it was despised in Spain) we have odds being quite close of 0. With a (-) sign before.