First of all, is this a real aunty, or just called aunty? If she's a blood relation, you're having an incestuous relationship.
Ok, that said, you should check to see if she's taking birth control of some kind, because she can still become pregnant. Unless a woman has gone through menopause, and her periods have ceased, she is still fertile, though probably not as much so as an 18 year old girl.
So, if you are both okay about committing incest, one of you needs to be preventing pregnancy, or you will get more than just the sexual appetites of a mature and experienced woman!!!!!!