She would be his Great Aunt
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Your sister's grandson is your great nephew. You are his great aunt (if you are a woman) or his great uncle (if you are a man).
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.
The son of your Uncle's son is your uncle's grandson. Your uncle's grandson is your first cousin, once removed.
Your great grandparents brothers and sisters are your great uncles and great aunts.
The grandson of your first cousin and your grandson are third cousins to eachother.
Your sister's great grandson is your great great nephew. You are his great great uncle, or great great aunt.