She would be your first cousin, once removed.
Edit: there is a chart (albeit a bit complicated) in Wikipedia - see related link.
Your cousin.
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.
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.
Great neice
Your grand-niece.
Yes
granddaughter
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.
Not sure but i guess it would be your great aunt's daughter
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).