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