You son and your niece are first cousins. Your son and your niece's son are first cousins, once removed.
Niece is to nephew as sister is to brother.
Son is to mother as nephew is to Aunt. Son is to farther as nephew is to Uncle. Daughter is to mother as niece is to Aunt Daughter is to farther as niece is to uncle.
She is your first cousin once removed, since your mother's nephew is your first cousin.
1st cousin
Your niece's son is your great nephew.He would be your grandnephew
Your son and your niece are first cousins.
Your niece's son's daughter is your great grand niece. Your niece's son is your grand nephew or your great nephew, whichever term you prefer.
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
Your granddaughter and your niece's son are second cousins.
Your niece's son and your daughter are first cousins, once removed.
The son of your great niece is your great great nephew.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your husband is great uncle to your niece's son (your great nephew).
Your aunt's son and her niece are first cousins to each other.
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.