Which is correct - great aunt or grand aunt?

The aunt of a great grand niece is her aunt - the sister of her mother or father.

The great grand niece has a great grand aunt, or great grand uncle, who is the sibling of one of her great grandparents.

In normal conversation, such a person would usually be called "Aunt," becasue adding all those "greats" and "grands" is cumbersome and would probably just make her feel old.