Take the item in to the store. When they see it hasn't been worn and still has the barcode and price label intact, you'll probably get a store credit for what it costs. It's hard to say no when a customer is standing there in the store. Good luck!! : /
Sorry but I believe the store is within its rights.
Your aunt chose something and bought it. That was the deal. If the thing she bought is faulty, she can take it back for a refund.
Changing your mind after you have bought something does not give you the right to change it or take it back (except under the Distance Selling Regulations). A shop CAN agree to an exchange or a refund but doesn't have to. Even then, the deal is between the shop and the person who bought the item.
As the recipient of a gift, you do not have any contractual rights. I think you need to work with your aunt on this one which I know is tricky because of the inconvenience and because you don't want to hurt her feelings about the gift. Sorry, but there isn't really a satisfactory way out of the situation.