Your wife's nephew is your nephew, but the English language has no relationship term for your nephew's wife. Just call her "my nephew's wife."
I suppose she would be your niece-in-law.
If it's your sister then your sister, if it's your brother's wife then you sister-in-law
Your nephew's wife is simply your nephew's wife. There is no special term in the English language for that relationship.
Great nephew
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
No. Your wife's niece is also your niece, but the husband of the niece is not related to you or to your wife.
He is their Uncle and they are nieces and nephews.
Your brother's wife's kids would be your nieces or nephews.
Second cousins.
niece in-law
If your great uncle is the brother of one of your grandparents, his wife is your great aunt but their nephews and nieces onher side are not related to you. Those nieces and nephews (and their children the great nieces and great nephews) have no ancestor who is also one of yourancestors.
The nephew of a wife and the nephew of her husband are not considered to be related to each other. However, both are nephews of the wife and the husband, and the wife and husband are aunt and uncle to both nephews.
Your brother's children would be your nieces and nephews.