Your father-in-law's brother's daughter is is your niece; also your spouse's niece.
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
A niece
A niece
Niece
A cousin.
Your niece.
natini
Your nephew?
Its would be your neice
Niece
The daughter of your brother is your niece.
I just say.. YO MAMA, but she is your niece.