She would be your great aunt.
Your great uncle's niece could be your mother, or your aunt. It is also possible that she is not related to you at all, as you might not share a common ancestor.
cousin
Cousin. Your parents brothers and sisters are aunts and uncles. and the aunt and uncles children are your cousins.
uncles aunts sholud both spend an equal amout of time together depending on their relationship
Dursley
You are their aunt or aunt by marriage.
Your aunt is your child's Great Aunt, so your child is her great niece or great nephew.
Sexual
You are First Cousins
You are not related to your uncles wife sister. However some may think an aunt
Tias = AuntsTios = Uncles
Since your mother and her sister have thesame aunts and uncles, your mother's sister's aunt is your grand aunt or, as some sat, your great aunt.