You are either her aunt or uncle depending upon your gender, and she is your niece.
If she is your sisters daughter then she is your neice :)
She is your niece.
She is your niece.
Niece.
Neice
niece
your Neice
Your niece
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.
Cousin - In - Law
Your 2nd cousin
your great aunt.