Your sister's daughter is your niece. If she had a son, it would be your nephew.The same applies if you have a brother, his children would also be your niece and nephew.
The age of your sister is irrelevant.
If your sister had a son it would be your nephew and depending on if your a boy or a girl you would be its aunt or uncle.
You would call her your niece. She would refer to you as aunt or uncle.
A Nephew.
niece
Your sister's son, is your nephew.
Your sister's son, is your nephew.
Your sister's son is called your nephew. You are his aunt (if you are a woman) or his uncle (if you are a man).
Your sister-in-law's grandson is your great nephew.
Your father's sister is you aunt. Her son is your first cousin. Her son's daughter (her granddaughter) is your first cousin, once removed.
The son of either your sister or your brother is your nephew. The English language does not make a distinction in that.
Your cousin
Your cousin.
Your Nephew
cousin
That would be his nephew.
Cousin