Your granddaughter could be your son's daughter, or your son's niece.
Your son is your granddaughter's father, or her uncle.
Your granddaughter and your niece's son are second cousins.
Your granddaughter and your sister's grandson are second cousins.
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 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.
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.
Your daughter's son is your grandson and you are his grandparent.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
You would be 2nd cousins, assuming that the brother's wife's granddaughter is also the brother's granddaughter. If the granddaughter is not the granddaughter of the brother, there is no relationship.
Your son's daughter is your granddaughter. Your sister is your granddaughter's great aunt.