I'm very sorry that your family and Mom are in this sad and difficult situation.
Did your Mother give anyone in your family, Power of Attorney(POA)? Since you think
your Mother may have Thyroid Cancer or at the very least is very unhealthy, this
would possibly be a good time to use it.
Does anyone involved, have a Lawyer? This would be the person to speak to about
any charges and what to do.
I doubt any of you would be charged with abuse. Perhaps video taping her saying that she does not want any medical treatment and that she does not blame anyone else for her decision. It is her decision if you cannot get through to her! Even without her knowledge of the video would probably be acceptable. Similar to a--Do Not Resuscitate Form (DNR)--form. I have such a form from the Hospital, for under certain circumstances. Also, you could ask someone with authority, a witness, so to speak, could be there without telling her ahead of time.
Although, I could not find the answer to your particular question, I did find this site that has other helpful information, if you have not seen it yet. There is a 'live chat' to explain your Mother's situation and to ask someone questions. If they cannot answer your questions, then they should know who you could contact. The services are supposed to be free of charge. Ask to make sure and print the questions/answers.
*Many years ago my Aunt was dying of Lung Cancer and all of the family tried to talk her into taking the treatment. She totally refused! When she was in the last stage, then she asked the Doctors for the Cancer treatment. Unfortunately, the Doctors then had to tell her that it was too late. It was very heartbreaking....my poor Uncle.
I wish far better for your Mother and Family! Take Care!