Your great aunt is your parent's aunt. A great aunt is the sister of one of your grandparents. An aunt-in-law is the sister of your spouse or partner, and a parent's sister is your aunt.
your great aunt is actually your parents' aunt. She is your grandparent's sister, making her your parent's aunt and your great aunt. Your parents' sister is your aunt, and your aunt in-law is your spouse's sister or brother's wife
She is your grandparent's sister, making her your parent's aunt and your great aunt. Your parents' sister is your aunt, and your aunt in-law is your spouse's sister or brother's wife