There is a public guardian whose job it is to handle situations like this. Look under city (or county) services for the name of your Public Guardian office. There may be a contact specifically for elder abuse. You can call a hotline and make an anonymous request for investigation. If you know the name of the mother's doctor to pass on the hotline that would be helpful.

Be prepared to be specific examplesof the behavior you have heard/observed so they know it is not just the caregivers blowing off the INCREDIBLE stress of taking care of an elderly parent with dementia.