I used to work in this field, and we would do pedigree analysis a lot on families, usually with some kind of disease.

Anyway, we would construct the family tree, plug it into the stats software and it would tell us: “No. These two people are not cousins, but something closer.”

We couldn’t ask the families, so we would swap a grandfather for a father, or a mother for a sister, and - hey presto - everything lines up.

This happened in about 1 in 10 families we studied. This type of things is way waaay more common than people think,