I am not sure that is a correct definition of incest, as it more commonly refer to breeding between family members or close relatives, with the definition of cl

I am not sure that is a correct definition of incest, as it more commonly refer to breeding between family members or close relatives, with the definition of close being up to the legal system in place. Just two people with a common ancestry would be a too board definition to be useful. But it is correct that 1 / 70,000,000,000 is the wrong numbers of the genetic contribution. In order to find the correct number one would have to determine the coefficient of relationship of the average couple. It is going to a much smaller number, but at the same time far from actually incest. That said, incest law in Italy is a bit weird. It is legal, except if it causes a scandal.