Learn English – What’s the name for Gary and Harry’s blood relationship

Imagine that Gary and Harry are these two guys in a hypothetical family,
described in English below and in this diagram represented by the green cells at the bottom:

As you see, Gary and Harry are related to each other by blood, so one is the
other’s something or other. What I want to know here is what that
something is. That way Gary could tell someone that “his _______
Harry stopped by the other day”, or vice versa.

In other words, is there a word — or words — that fit these two blanks?

• Gary is Harry’s ________.
• Harry is Gary’s ________.

This might be the same something, like with cousins, or it might be
different somethings, like uncle and nephew.

Here’s their family tree, which starts with two couples, Andy and Betty
on one hand and Charlie and Daisy on the other, in which none of the
four is related to any other of the four by blood, only by marriage alone.

1. Andy and Betty marry and have a son Eddie.
2. Charlie and Daisy marry and have a daughter Fanny.
3. Later, Andy divorces Betty and marries Charlie and Daisy’s daughter Fanny, and then Andy and Fanny have a son Gary together.
4. Also later, Daisy divorces Charlie and marries Andy and Betty’s son Eddie, and then Daisy and Eddie have a son Harry together.

That means that Gary is Andy’s son and Daisy’s grandson, while Harry is Daisy’s son and Andy’s grandson. Gary’s half-brother Eddie is Harry’s father, while Harry’s half-sister Fanny is Gary’s mother.

Given that Andy is Gary’s father and Harry’s grandfather, and Daisy is Harry’s mother and Gary’s grandmother, Gary and Harry are definitely related to each other — but how?

• Gary is Harry’s ________.
• Harry is Gary’s ________.

While I realize that not all extended, by-marriage family relationships
have a name for them in English that do have a word for them in other
cultures, that’s not what I’m talking about here. These are for people who
actually share blood with each other, so you would think some
combination of cousin or uncle or nephew or grandson would work here.

But what?

Here’s that diagram again in case in helps to see it closer to the English description.

_{This idea and original diagram from F.M. Lancaster’s Genetic and Quantitative Aspects of Genealogy: Types of Collateral Relationships}