nephew
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
Your cousin.
nephew!
We would infact, cll our sisters son your nephew and they would call you either auntie or uncle depending on your sex.
Our cousins are our mothers sisters son :)
My sister's son is my nephew.
My sister's son is my nephew.
If your sister had a son, then he is your nephew.
The answer is: herself or one of her sisters. Her mother's son is her brother, and she and any female siblings would be her brother's son's aunt.
You would call your sister's son your nefu and her daughter neice.
Niece if it's a female, and nephew if it's a male.