Your daughter is your sister's niece. Your daughter's son is her great nephew.
You would call your sister's daughter your niece.
Grandson :)
Your sister's son will call you his uncle, if you are a man, or his aunt, if you are a woman.
She is your third cousin.
You call him your nephew. (pronouced nes-few)
Your grandson.
Your grandson.
Your daughter's husband is your son-in-law.
Your sister's son is your nephew. You are his aunt or uncle.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
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.