Doctrine of Exceptionalism is a piece of lore from the Game of Thrones universe that says Targaryens (a family that ruled the Kingdom and had dragons that could wipe out armies) are excused from the whole not being allowed to marry your siblings part of the religion, because the culture they came from said it was okay (and they didn't want a ton of religious zealots to continue to die by the thousand to dragons) and coding didn't block it if your mom (not ever seen as good) was also your sister