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a nephew's kid to his aunt's kids would be their cousin.
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
Her grand great Aunt.
Your grandfather's brother's son is your first cousin, once removed. His mother is your great aunt (or grand aunt), but her sister is not related to you.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Her grand great Aunt.
While some people prefer the terms grand aunt or grand uncle, parallel to grandmother or grandfather, the terms great aunt and great uncle are widely used and accepted, and are functionally the same.Your great aunt is the sister of one of your grandparents, which makes you a grand niece or grand nephew (great niece or great nephew). The son of said grand aunt is a first cousin of one of your parents. The son's daughter is first cousin once removed to your parent, and second cousin to you.
Your niece's son is your grand nephew, and you are his great aunt.
Your aunt's daughter is your first cousin, as is your aunt's son.
Presuming your son is also your wife's son, the your wife's aunt is your son's great aunt and he is is her great nephew.
Your aunt's son and her niece are first cousins to each other.
Your grandmother's sister is your great aunt, her grand children are your second cousins twice removed. Genetically speaking this is barely a relationship - but if you were raised together as cousins the familiar bond can be tight.