Well, darling, your stepmother's niece is not related to you by blood at all. So technically, you have no relation to her whatsoever. She's just a distant relative by marriage, like a stranger you happen to know through your stepmother. So, feel free to keep it cordial, but don't worry about sharing any DNA with her.
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.
cousins
your niece
Your Aunt's niece is you (if you are female), your sister, or your First Cousin.
that would be your niece or if you want to be fussy then your niece in law!
Your brother's daughter is your niece. Her daughter is your great niece.
Niece
niece/nephew
great nephew