nothing
Your sisters grandson would be your grandnephew.
Your granddaughter and your sister's grandson are second cousins.
Your grand-niece.
Your mothers sisters Granddaughter is your second cousin
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.
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
The relationship is that of a great great great grand uncle and great great great grand niece
WHAT relationship* is your Gramps to your great granddaughter? pretty tricky but (distant sisters one off)
Great neice
Yes
Your great great grandmother's sister's great great grandson is your third cousin, twice removed.
Sisters