Leisa is the son of the Pope and the Duchess, so he isn't the product of an incestuous relationship
He is the son of the pope and duchess but still the duchess is a child of inc*st who just so happened that was not affected by the disorder unlike his brother sayan. Though you are correct that it widens the percentage of leisa inheriting it but sayan said so himself that leisa could have it. So far we have not seen leisa acting like sayan or carmide but we have seen little of his pov.
Lessens the percentage*
Just opening some discussion but children from inc*stous rs like sayan and carmide except iss, who inherit its pychological illness with the voices/delusions really does make the story complex. I just hope leshia does not inherit the disorder.