Your great grandmother's sister is your great great aunt and your son's great great great-aunt. He is her great great great nephew.
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.
Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
You would be a great uncle, so they would be your great nephew
Your sister's son's daughter is your sister's granddaughter and your great niece. (Some would say "grand niece.") You are her great uncle or great aunt.
Your daughter and your sister's son are first cousins to each other. Your sister's son's daughter (her granddaughter) and your daughter are first cousins, once removed.
Your sister-in-law's son is your nephew.
Your husband's sister's son is your nephew, just as your sister's son is your nephew.
The sister's son would be the nephew of her brother.
Grand-Nephew