Son has POA, neglects his mother's health. Can I get POA? I'm her sister.

If your mother is competent to sign ( it may have to be proven in the presence of an attorney that she is competent to sign) she can transfer POA to anyone she chooses. However, if her dementia is too advanced, you may have to file for guardianship. Either way, you need to get an attorney involved because things are going to get ugly.