She would be your son's second cousin.
The son of your great niece is your great great nephew. You are his great great uncle.
Your niece's son is your great nephew. You are his great aunt (if you are a woman) or his great uncle (if you are a man).
Your niece's son is your great nephew.He would be your grandnephew
my great grand-daughter
Your niece's son is your great nephew. You are his great aunt or great uncle.
You are their great-uncle/aunt.
There probably isn't a relationship. You do not share a common ancestor.
In the United States there are no "legal relationship" between relatives, because no laws define these relationship. These are all customary terms. The child of one's niece or nephew is one's great niece (if a girl) or great nephew (if a boy). The elder relative is a great aunt or great uncle.
They're related to you
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.