Your niece.
If she is your sisters daughter then she is your neice :)
She is your niece.
niece
Niece.
Neice
She is your niece.
You would call your sister's son your nefu and her daughter neice.
your Neice
Your niece
Your sister's daughter is your niece. You are her aunt if you are female, or her uncle if you are male.
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.
Your 2nd cousin