Your mother's nephew is your First Cousin.
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The English language has no special name for a nephew's wife and does not consider you to be related to you. You would simply call her "my nephew's wife."
Your niece and nephew.
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.
The grandson of your nephew is your great great nephew, but few people use that relationship as a form of address. Usually they call relatives by name or business title, not by relationship.
The English language does not contain a specific term to describe a relationship between you and your nephew's wife. The culture does not consider her to be related to you. Most aunts and uncles would merely call her "my nephew's wife."