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
Your granddaughter and your niece's son are second cousins.