Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
I believe they would be great aunts or great uncles.
Well, they are related but a name has not been given to that yet! :)
a nephew's kid to his aunt's kids would be their cousin.
You're a Great Aunt to your nephew's son, who is your great nephew.
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
Your nephew's children are your great nieces (girls) and great nephews (boys). You are their great uncle or great aunt, depending on your sex.
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
You don't, simple as that. You could say cousins if it's that important, or "my aunt's nieces and nephews on the other side," but there isn't an actual term for that except your "uncle's wife's siblings children."
You are your great nephew's great uncle or great aunt, depending on your sex.
There is no special term in the English language to describe a relationship between you and your nephew's wife. To her, you are just "my husband's uncle." If she has children with your nephew, they will be your great nephews and great nieces and you will be their great uncle or great aunt.
Your grandfather's great nephew is your second cousin.