Wat the heck in this question yaar? its maternal uncle. you are uncle your son is first cousin to your sisters his aunt sons or daugters a : a child of one's uncle or aunt b : a relative descended...
Your sister's son is your nephew.
Your son's daughter is your granddaughter.
Your son and your sister's son are first cousins to each other.
Your son's daughter and your sister's son are first...
Your sister's son and your son are first cousins.Your sister's son and your grandson (your daughter's son) are first cousins, once removed, to each other.