Every parent is occasionally faced with questions from their children for which they simply don’t know the answer. This is why the Internet exists, and why I’m glad I waited until 1993 to become a parent. My son is questioning this whole fifth cousin, twice removed thing, and I was forced to confess I don’t really know how all that goes. Specifically, one of my cousins is about to marry my son’s friend’s uncle, and my son wants to know how he and his friend will be related.
The short answer is that they won’t. Generically, they will be referred to as having an “affinal relationship”. That is, you can draw a path between them on a family tree, but they do not share a common ancestor.
But I was still curious as to what my son’s relationship to the bride really is. She is the great-granddaughter of my great-grandparents. That’s a cousin of some sort, but what are the naming rules? Fortunately, I’ve discovered an easy way. Start by drawing a tree from the most recent common ancestor to both people. Now count the number of steps from each person to the common ancestor. For my son that is 4 steps, and for the bride-to-be it is 3 steps. The difference in these 2 numbers is the “removed” part. So they will be some sort of cousins, once removed. Then take the smaller number (3) and subtract 1. This is the cousin level. So the bride and my son are second cousins, once removed.
Children of the new marriage (which, according to rumors from the bridal shower, seem unlikely) will be my son’s third cousins and his friend’s first cousins. But that’s as close as they’ll ever get to each other on the family tree.
We will now resume your previously scheduled life, already in progress.