Your granddaughter and your sister's grandson are second cousins.
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 could be your son's daughter, or your son's niece.
Your granddaughter and your niece's son are second cousins.
Yes, she did. She had the relationship of a mother that a mother has with her son. As most mothers do.
YesThe son and his sister = 1 son, 1 sisterThe sons and their sister = many sons, 1 sisterThe son and his sisters = 1 son, many sistersThe sons and their sisters = many sons, many sistersThough in reality this is a strange way to say it. More normal would be "The sons and daughters".
step-granddaughter
Your son and your first cousin's granddaughter are second cousins, once removed, to each other.
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
It depends - he could be either her son or her nephew.
There is no relationship. The two of you do not share a common ancestor.
You would be his Great Uncle or Great Aunt He in turn would be your Great Nephew.