Unrelated is an example.
nephew
There is no antonym.for nephew. The female relation of the same status is 'Niece'. Their relationship to the previous generation is that they are their father's/mother's brother(Uncle) or sister(Aunt).
There is no "opposite" of a nephew, unless you say that since a nephew is male, the opposite is a niece, who is female.
Your nephew's nephew is the son of one of your nephew's brothers or sisters. He is your great nephew.
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
Your nephew's son is your grand nephew or great nephew.
Your mother's (mom) nephew is your answer.
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.
Niece would be an opposite as a niece is female and a nephew is male.
In English language terms for family relationships, there is no relationship called "nephew once removed." The son of your nephew is your great nephew.
Your nephew's son is your great nephew.