Your sister's daughter and your son are first cousins to each other.Your sister's daughter and your son's daughter are first cousins, once removed to each other.Your son's daughter and your sister's...
Your sister's daughter is your niece, and her son is your grand nephew (or great nephew). You are his great uncle (if you are a man) or great aunt (if you are a woman).
Your son and your sister's daughter are first cousins. Your son and your sister's daughter's son (your sister's grandson) are first cousins, once removed, to each other.