Because they are both female, and that is the way the English language does it.
Your brothers ( or sisters ) daughter is your ( niece )
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
Your brother or sister's daughter is called 'your niece.'
Your sister's daughter is your niece and you are her aunt (if you are a woman) or her uncle (if you are a man).
Your sister's daughter is your niece. If you are a woman, you are your niece's aunt; if you are a man, you are your niece's uncle. Your sister's son is your nephew.
If she is your sisters daughter then she is your neice :)
She is your niece.
yes it is
niece
Niece.
She is your niece.
She would be your niece.