Your sister's son will call you his uncle, if you are a man, or his aunt, if you are a woman.
You call him your nephew. (pronouced nes-few)
Your sister's son is your nephew. You are his aunt or uncle.
If your sister had a son, then he is your nephew.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your father's sister's son (or daughter) is your first cousin.
He would call her his aunt.
Newphew
Nephew
Your nephew.
Your daughter is your sister's niece. Your daughter's son is her great nephew.
your co son.