Your aunt's son and her niece are first cousins to each other.
Your niece's son is your great nephew.He would be your grandnephew
Your granddaughter and your niece's son are second cousins.
your 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).
You are their great-uncle/aunt.
She would be your son's second cousin.
your niece
They're related to you
Your granddaughter could be your son's daughter, or your son's niece.
There probably isn't a relationship. You do not share a common ancestor.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.