Zeroth cousins share the same parents so it’s the same as siblings.

Continuing that, we get that you are your own “negative first cousin” (i.e. sharing yourself) and your spouse is your “negative second cousin” (i.e. sharing a child).

However, it doesn’t work that way. The degree of cousinhood is determined by how many generations there are between the common ancestor and the cousins. If the cousins belong to different generations, we take the closest cousin.

First cousins make sense because there’s one generation between you and your common ancestor (your grandparents).

Zeroth cousins also make sense because there’s no generation between you and your common ancestor (your parents).

But negative first cousins doesn’t make sense. You can’t have -1 generations between you and your common ancestor.

The common ancestor between “you” and “yourself (which is you)” are your parents. As there are zero generations between you and your parents, you are your own zeroth cousin.

Even if you allow yourself to be your common ancestor, the amount of generations between you and yourself is still zero. So you’re still your own zeroth cousin.

So if you want to describe everyone’s relation using “xth cousin x removed”, here’s how you do it:

0th cousin - Self, brother, sister

0th cousin once removed - Mother, father, son, daughter, aunt, uncle, niece, nephew

0th cousin twice removed - Grandparent, grandchild, great-aunt, great-uncle, great-niece, great-nephew

0th cousin n times removed - Add n-2 “greats” to above

1st cousin - The child of your uncle or aunt

1st cousin once removed - The 1st cousin of your parent or the child of your 1st cousin

1st cousin twice removed - The 1st cousin of your grandparent or the grandchild of your 1st cousin

2nd cousin - The grandchild of your great-aunt or great-uncle

2nd cousin once removed - The 2nd cousin of your parent or the child of your 2nd cousin

2nd cousin twice removed - The 2nd cousin of your grandparent or the grandchild of your 2nd cousin

3rd cousin - The great-grandchild of your great-great-aunt or great-great-uncle