Your sister is the great-aunt (also written great aunt) of your granddaughter.
Your sister's grandchildren are your great nieces (girls) and great nephews (boys). You are their great aunt (if you are a woman) or great uncle (if you are a man).
That would be her great uncle I suppose. Common sense.
Your sister's child and your child are first cousins. Your sister's child and your grandchild are first cousins, once removed.
Your sister's daughter is your niece. They are your children's first cousin. Your grandchildren would be first cousins once removed.
Your sister is her great grandaunt.
Greataunt
Your grand-niece.
Your sisters grandson would be your grandnephew.
Your mothers sisters Granddaughter is your second cousin
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.
WHAT relationship* is your Gramps to your great granddaughter? pretty tricky but (distant sisters one off)
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.
Yes
Great neice
The relationship is that of a great great great grand uncle and great great great grand niece