If your sister had a son, then he is your nephew.
nephew
Your cousin.
Our cousins are our mothers sisters son :)
We would infact, cll our sisters son your nephew and they would call you either auntie or uncle depending on your sex.
My sister's son is my nephew.
cousin brother
Niece and nephew
My sister's son is my nephew.
Your sisters son is your nephew.
You would call your sister's son your nefu and her daughter neice.
Your father's sister is your aunt. Her son is your first cousin. Her grandson is your first cousin, once removed.
Your uncle's son would be your cousin. Your parents' brothers and sisters are your aunts and uncles. All of their kids, regardless of gender, are your cousins.