Great aunt or uncle.
Your grandson is the great-nephew of your brother.
The English language has no relationship name for the wife of your great niece or great nephew.
Your son and your nephew are first cousins to each other. Their daughters are second cousins to each other.
Neice or nephew no dummy its nephew
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.
Your dad's nephew is your first cousin, his children are your first cousins once removed.
Your sister's son is your nephew.
Grand-Nephew
Your nephew as well...
You're a Great Aunt to your nephew's son, who is your great nephew.
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."