East gate Zara is refusing to exchange the shoes

My younger sister just bought the ankle boots on Friday 12 March 2021 and unfortunately the other shoe look like they were using it for people to fit and it happen to be bigger than the other and she noticed that when she was away and wearing the same shoes. The shoes were not comfortable and now she is having blisters. She went back with the shoes to Zara but unfortunately they refused to help her. They stated that she wore them and who does he think will now buy them. She was assisted by Thabile Ndlovu at East Gate Zara. She was shouting at my sister and told her to buy another pair of shoes and leave them alone. The question is, is this how they are treating us. I am so angry because now they expect me to buy the same type of shoes. Please help my younger sister.