Legal Question in Elder Law in Massachusetts

Given your mother's condition a Probate Court in MA should grant her Guardianship and you should consent to it as well as your mother being removed to ME.

Under a Medical Proxy you can only do a limited number of things when your mother is unable to consent to them. Even if you have a Durable Power of Attorney, your powers are not the same as Guardian. I would also suggest in the event your sister is not able to continue as Guardian, you ask that you be approved as the next Guardian for her.

But as to your liability for allowing your sister to remove your mother and take her to ME, you will have some limited liability to your mother and anyone she might injure, if she is a danger to people when she is in a dimentia state.

Only if your mother objects to being in your sister's care will there be an issue. I am assuming this is being done so your sister can look in on her at another facility.