Technnically, she is not related as you do not share a common ancestor. She would refer to you as Aunt and she is your niece.
Whether he is your first, second or fifth husband, his niece is your niece while you are married to him.
Your husband's niece is your niece, too. You and your husband are her aunt and uncle. Your brother is not related to a niece of yours who is the daughter of your husband's sibling.
Your husband is great uncle to your niece's son (your great nephew).
Your sister's daughter is your niece and you are either her aunt or her uncle. The English language has no term for your niece's husband, other than calling him "niece's husband."
cousin in law.
Your sister's husband is your brother-in-law. His niece is a child of one of his siblings, and is also a niece of your sister by marriage. There is no relation to you.
You are an aunt/uncle-in-law! Congratulations!! You really are; because you're an aunt/uncle to your niece you are now an aunt/niece by marriage to her husband.
She has no relationship with you. You do not share a common ancestor. You technically are not even related to your niece's husband.
You are legally her aunt through marriage.
your 2nd niece...!
Your children and your husband's niece are first cousins. There is a difference of one generation between them. That makes them first cousins once removed.
Technnically, she is not related as you do not share a common ancestor. She would refer to you as Aunt and she is your niece. Whether he is your first, second or fifth husband, his niece is your niece while you are married to him.
2nd cousin or 2nd cousin in law.