Niece is already the answer - A male niece is a nephew.
Your brother's (or your sister's) son is your nephew and you are his aunt if you are a female or his uncle if you are a male.
You are your nephew's uncle if you are male, and his aunt if you are female.
Niece would be the female counterpart for a nephew.
Male niece is female
There is no "opposite" of a nephew, unless you say that since a nephew is male, the opposite is a niece, who is female.
The female nephew is called the neice.
a nephew
You are her niece or nephew (niece if female, nephew if male) and she is your aunt.
female.male.
Ebenezer Scrooge's nephew's name is Fred
Your father's brother is your uncle. You are his niece (if you are female) or his nephew (if you are male).