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.
Yes
Great neice
Your grand-niece.
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 granddaughter is your great niece and you are her great aunt (if you are a woman) or great uncle (if you are a man).
Your her great great aunt!
WHAT relationship* is your Gramps to your great granddaughter? pretty tricky but (distant sisters one off)
How about your sisters brother's mother's father's granddaughter!
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.