They are of no blood relation. But they are cousins by marriage or 'step-cousins' as such.
What is my godfathers niece to me
Your niece's brother is your nephew.
Your son and your niece are first cousins.
Nothing. Your niece in law. Your brother in laws niece.
your niece
cousins
Your Aunt's niece is you (if you are female), your sister, or your First Cousin.
Your brother's daughter is your niece. Her daughter is your great niece.
that would be your niece or if you want to be fussy then your niece in law!
Niece
great nephew
Great-niece.