Well, still it is very, verrrrrrrrrrrrrrry unlikely to see. At least not openly. But as you know, political marriage between the close relative was not so uncommon, as the history of Habsburg - after spread out the family member through the royal family of europe, after some centuries many royal family are close relatives of each other. Although they try to avoid of marriage between too close relatives, but avunculate marriage or cousin marriage was sometimes allowed by the pope, by needs of political reason.
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Anyway, the main reason to open the thread is, what's the base of the number? Is the number means step to the family-3? So a person's parents/childs are 1-3=-2, siblings and grandparents are 2-3=-1, uncle/aunt/niece/nephew are 3-3=0, cousins area 4-3=1, so on.