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 aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
You would be his Great Uncle or Great Aunt He in turn would be your Great Nephew.
Your daughter's son is your grandson and you are his grandparent.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
Your sister's granddaughter is your great niece and you are her great aunt (if you are a woman) or great uncle (if you are a man).
Your granddaughter and your sister's grandson are second cousins.
Your granddaughter could be your son's daughter, or your son's niece.
Your granddaughter and your niece's son are second cousins.
Your grand-niece.
Your sisters grandson would be your grandnephew.
Your mothers sisters Granddaughter is your second cousin
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.
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.