Your grandson is your sister's great nephew and she is his great aunt.
Second cousin.
She would be his Great Aunt
The great grandson of your sister is your great great nephew. You are his great great aunt or great great uncle.
Second cousin, once removed
Second cousin, once removed. Your parent on that side would be a second cousin to the grandson.
No, Brother-Sister relation
The grandson of your grandmother's sister is your Second Cousin.
The grand children of your granmother's sister are your Second Cousins because you share one or more great-grandparents.
There is no relation between you and your grandmother's cousin-in-law's grandson.
Your sister would be your grandson's great aunt.
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your grandmother's sister's grandson is your second cousin.