I call my grandma's aunt, Great Aunt. Maybe that's what you call her, but she is of course your great-great-aunt.
Your aunt's child, son or daughter, is your First Cousin. The same is true for your uncle's children.
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
The answer is: herself or one of her sisters. Her mother's son is her brother, and she and any female siblings would be her brother's son's aunt.
Your father's aunt's son is your father's first cousin and your first cousin, once removed.
I'd normally call him by his first name. And the son of your aunt is your first cousin.
The name given to the son of your aunt is first cousin or simply cousin.
Your aunt's son and your children are first cousins to each other.
He would call her his aunt.
He would call your aunt his great aunt. In informal situations, great aunts are sometimes called, simply aunt.
Your sister's son will call you his uncle, if you are a man, or his aunt, if you are a woman.
He is your cousin
Great Aunt.
You will be his aunt.
Your sister's son is your nephew. You are his aunt or uncle.
Great Aunt.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.