How to deal with a sibling who has autism and who constantly makes noise?

I would suggest going somewhere quiet for a while to just relax and be by yourself. For example a library or coffee shop. Another thing to do is to try to find something that will soothe your sister and make her calm so she doesn't make noises. Reading to her, going for a walk, taking her to the park, playing music for her, there has to be something that will calm her. You can maybe even try, if your parents will go for it, getting her a pet, like a dog or cat.