Most likely, your cousin
Another answer:Your niece's first cousin would be your niece or nephew (or your son or daughter).
Explanation:
Your parents have 3 children: you, your brother, and your sister.
Your brother has a daughter. She is your niece.
Your sister has a son or daughter. He or she is your niece's first cousin. He or she is also your nephew or niece.
You have a daughter. She is your niece's first cousin.
It helps if you draw this out on paper.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Second Cousin
You would be a first cousin by marriage, in other words, there is no direct relationship between you.
Your cousin's cousin is no relation to you.
Your mother's first cousin is your first cousin, once removed.
Your mother's first cousin is your first cousin once removed.
Your mother's first cousin is your first cousin, once removed
The child of your first cousin is your first cousin, once removed.
The wife of your first cousin is not related to you.
Your Aunt's first cousin may be your second cousin, onve removed or may be no relation to you at all.
Your Aunt's first cousin may be your second cousin, onve removed or may be no relation to you at all.
Cousin once removed.