Your sister's son is your nephew.
Neice or nephew no dummy its nephew
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
The boy is your nephew
Your son and your sisters child are first cousins. The granddaughter is out of synch by a generation. That will make them first cousins once removed.
Your granddaughter and your sister's grandson are second cousins.
Yes, she did. She had the relationship of a mother that a mother has with her son. As most mothers do.
Your younger brother's wife's sister's daughter would be your son's cousin.
Stepson; Son in law; Brother?
Your son's wife is your daughter-in-law. Her brother is your son's brother-in-law, but the English language has no word for a relationship between you and him. You are not considered to be related to him.