Your sister's grand daughter is your grand niece or great niece. You are her great aunt or great uncle.
Great Aunt/Uncle
Great nephew.
Great niece
Great nephew
grand nice
My
Great neice
WHAT relationship* is your Gramps to your great granddaughter? pretty tricky but (distant sisters one off)
your niece
Your uncle's granddaughter and your child are second cousins.
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
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 granddaughter and grand niece are second cousins to each other.
Your great nephew and your granddaughter are second cousins to each other.
Yes
Your grand-niece.