The granddaughter of your great great grandmother may be one of several different relatives. She can be your:
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.
Your granddaughter and your niece's son are second cousins.
Your sisters grandson would be your grandnephew.
Your granddaughter could be your son's daughter, or your son's niece.
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 brother's daughter is your niece and his granddaughter is your great niece.
Your first cousin's granddaughter is your first cousin, twice removed.