A husband's niece is the daughter of his brother or sister. A wife's niece is the daughter of her brother or sister. In general, the two nieces are not related to each other.
The only way the two nieces could be related would be if the husband and wife were related in some way in addition to their relationship through marriage, for example, if they were also cousins to each other.
The English language has no relationship name for the wife of your great niece or great nephew.
No. Your wife's niece is also your niece, but the husband of the niece is not related to you or to your wife.
Probably your niece-in-law.
either your niece's husband, or your nephew, or your nephew in law, etc
that would be your niece or if you want to be fussy then your niece in law!
Your wife's niece is also your niece, and her daughter is your great niece. It is to be hoped that you do not have a relationship WITH her, merely TO her.
bigamy
He is very old and has multiple husbands/wifes
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.
Great grand niece
great niece
Your niece's child is your great niece if she is a girl, or your great nephew if he is a boy.