You are not related to your niece's husband, although some would call him your nephew by marriage. But that is a relatively recent construction and not used by many.
You could also call him your brother's (or sister's) son-in-law.
You are their aunt.
They are your nieces and nephews.
Even thought you not related in genealogy sense you are related by marriage and you can say they are newphew and nieces in law.
Children of your siblings are nieces and nephews. Children of your spouse's siblings are still called nieces and nephews, but you are not related to them by blood.
When a couple is married and the wife has nieces who are daughters of her brothers or sisters, those nieces are also the nieces of her husband. So your great grandmother's sister's husband could have great nieces who are related to you and also great nieces who are not related to you.Note that your great grandmother's sister is your great great aunt.The great nieces who are related to you are the grandchildren of the siblings of your great great aunt.Those could be your:parentauntfirst cousins, once removedsecond cousins, once removed
Your husband's nephew is also your nephew, and you are his aunt. Remember that if your father has a married brother, the brother is your uncle and the brother's wife is your aunt , with no mention of one being related by marriage and the other not. They are simply your aunt and uncle. The same applies to your relationship to your husband's sibling's children. You are simply their aunt.
Your sister's husband is your brother-in-law. Your brother-in-law's brother is not related to you and neither are his children. But he is your sister's brother-in-law and his children are your sister's nieces and nephews.
He is their Uncle and they are nieces and nephews.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
The children of your husband's sister are your nieces or nephews, and you are their aunt, just as he is their uncle.
Your sister's husband is your children's uncle. They are his nieces and nephews.
You do not have a niece-in-law. Some of your nieces and nephews are that because they are the children of your husband's siblings. They are your nieces and nephews and you and your husband together are their "aunt and uncle."
They are not related.