There is no specific term in the English language to describe a relationship between you and your nephew's wife. Most people would say you are not related. If you must use some phrase, the phrase "my nephew's wife" is probably the clearest and most direct.
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
Great nephew
He is their Uncle and they are nieces and nephews.
Your nephews' children would be your children's cousins. They share a common grandparent, as their parents (your nephews) are siblings to your children’s parents. This familial relationship creates a bond among them, typically fostering a close connection within the extended family.
Second cousins.
Your brother-in-law's children are your nephews and nieces.
Your brother's children would be your nieces and nephews.
If it's your sister then your sister, if it's your brother's wife then you sister-in-law
The English language has no relationship name for the wife of your great niece or great nephew.
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your borther-in-law is your grandchildren's great uncle. They are his great nieces and great 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."