Assuming that you are a female and that your mother actually has a brother, then yes, you are your mother's brother's niece.
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
Your niece would be your mothers granddaughter.
your step niece,,, if it was your brothers own daughter it would be your niece, since its his stepdaughter, than its your step niece.
Niece
A niece
Your niece.
niece
A niece
Niece. Your mother has children, you and your sister. So your mother's daughter is your sister. When she has a daughter, this is your niece.
niece
Ivonette
Your niece.