There is no "opposite" of a nephew, unless you say that since a nephew is male, the opposite is a niece, who is female.
There is no antonym for nephew
nephew
Unrelated is an example.
Your nephew's nephew is the son of one of your nephew's brothers or sisters. He is your great nephew.
Your mother's (mom) nephew is your answer.
Your nephew's son is your grand nephew or great nephew.
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
Your wife's nephew is your nephew and his son is your wife's, and your, great nephew.
The male counterpart of a niece is a nephew.
Your nephew's son is your great nephew.
Your nephew's son is your great nephew.
Niece would be an opposite as a niece is female and a nephew is male.