Help! How do you stop someone being obsessed with going to the toilet?

Your sister may be suffering from Irritable Bowel Syndrome, more commonly known as IBS. Even though it isn't a serious illness in itself, it affects a person's quality of life. In your sister's case it is obviously not only affecting her, but the whole family. Persuade her to see a doctor about the problem and he may prescribe suitable pills to help her.