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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 sisters grandson would be your grandnephew.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
Your 1st cousin's granddaughter would be your daughter's niece.
Your uncle's great granddaughter is your first cousin, twice removed.
Your mothers sisters Granddaughter is your second cousin
No relationship at all, you don't share a common ancestor.
Your sister is her great grandaunt.
Your niece would be your mothers granddaughter.
Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
I believe they would be great aunts or great uncles.