Relationships were like kinslaying in Westeros. It was okay between cousins. The wizarding world was no different, and there were many arguments about how inbreeding had affected magical society, yet the often married their relatives, both close and distant. But on the other hand, in Westeros, all of the nobility had been intermarrying for thousands of years, and valyrians wed sister to brother for millennia without any trouble. If things here were the same as on earth, the nobility here would long be unable to reproduce, and if they did manage, they would look like something straight out of a horror movie, an even more grotesque version of the last Gaunts. For whatever reason, that wasn't the case. Jon heavily suspected some sort of magic at play.