It depends - he could be either her son or her nephew.
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 sister is her great grandaunt.
Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
ya'qub.
Your granddaughter could be your son's daughter, or your son's niece.
Your granddaughter and your niece's son are second cousins.
Your sisters grandson would be your grandnephew.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
Grandniece
Your brothers great granddaughter is your great great niece.
Your granddaughter and your sister's grandson are second cousins.
Your first cousin's granddaughter is your first cousin, twice removed.