Attitude Help

My three year old has been having this attitude problem with my sister lately. My sis suffers from extreme eczema where her skin is visibly dry flaky and falls off. At times when I ask my sis to help my daughter with the washroom or changing her clothes my LO says “no I can’t because her eczema is going to touch me”. I have seen a few times that it has hurt my sis and I had a talk with my LO to help her understand. Yet she still continues to say so and I find it quite rude of her. Apart from that she has a really good relationship with my sister. I’ve tried telling her about how much she suffers from her skin and that she won’t even touch her while helping but she doesn’t accept it.

Is there any other route I can take to help her understand and not say that anymore?