Your great-niece.
Grand-Nephew
If your sister had a son, then he is your nephew.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
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."
You are your nephew's uncle if you are male, and his aunt if you are female.
Your mother's nephew is your First Cousin.
You hand it to your oldest daughter
your second nephew
Your grandson is the great-nephew of your brother.
Either nephew or nephew in law.
Your son and your nephew are first cousins to each other. Their daughters are second cousins to each other.
My sister's son is my nephew.