Hi carbon12, I'm very sorry for all that you and your sister are going through. I think consulting a domestic abuse hotline, or contacting adult protective services are great suggestions. I would also like to suggest you speak to an attorney. You could speak to one in your own state, you wouldn't necessarily need to speak to one there. If you can afford to hire one, great! If not, most cities do have some free or low cost legal services, often attached to law schools, where you could speak to someone. They would be able to research laws in the state where your sister lives, and advise you on how to proceed.
I know you are worried you will make things worse for your sister, but the only way her situation is going to change is if you or she take action. I hope you will be brave enough to do so! I wish you all the best in this. Thank you for sharing with us.