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.
Your grandson.
You call him your nephew. (pronouced nes-few)
Your grandson.
Your sister's son is your nephew. You are his aunt or uncle.
Your daughter's husband is your son-in-law.
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.