2nd cousin or 2nd cousin in law.
A nephew is a cousin to his aunt's son.
Your son and your aunt's son are first cousins once removed to each other.
your cousin
Your great aunt's son is your parent's first cousin and your first your cousin, once removed.
Your mother's aunt's husband's sister is your mother's aunts sister-in-law but is not related to you. Her son is not related to you either.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
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 niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your grand niece and your grand child share a common great grandparent. There is no generation difference. That would make them second cousins.
They would be cousins, once removed.
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.