You brother's daughter is your niece, just as your sister's daughter is your niece. The English language does not have different terms for daughter of brother and sister.
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
Niece
A niece
A niece
natini
Your niece.
A cousin.
Its would be your neice
Niece
Your nephew?
The daughter of your brother is your niece.
I just say.. YO MAMA, but she is your niece.