Your grand niece's son is your great grand nephew, or, some would say, your great great nephew.
You are his great grand aunt or great grand uncle, or, some would say, great great aunt or great great uncle.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Second Cousin
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Your niece's son is your grand nephew, and you are his great aunt.
Your mother's nieces' children are your mother's grand nephews or grand nieces. They are your first cousins, once removed.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
The vast bulk of it went to nieces, nephews, grand nieces and grand nephews. Anthony Dion Fay received only $5,000, according to Victoria Wilson.
Grand niece.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.
You are her great-great Aunt (or great grand Aunt, depending upon your preferred nomenclature). She is your great great niece.
Great-niece.