If she is your sisters daughter then she is your neice :)
niece
She is your niece.
niece
Neice
Niece.
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.
You would call your sister's son your nefu and her daughter neice.