How do I protect my mom from being manipulated by her mentally ill daughter?

Yes you hit a nerve and from the lack of further information, I thought these questions did need answering.

Your own wording protection from a mentally ill daughter led me to think you must be referring to someone other than yourself because I've seen such objectifying of siblings and a parent's spouse. Thus, since you did not say you were the daughter with the mental illness, I wondered if there was another daughter. So, I gather then you are the one with a mental illness and are wondering how to protect your mother from yourself? Did I get that right?

Yes, I would see a mental health professional.

Your response tells me a lot. There was no mention of an "other daughter." I asked this question looking for possible answers or support, not judgement based on speculation. I am sorry this touched a nerve with you. Perhaps this question is better asked of a health care professional. Please pay attention to the admonishion under the "ADD YOUR ANSWER" window.

So your mother's 'other daughter' has a mental illness?

What is the diagnosis which your sister, half sister, or step-sister has?

Does your mother aware of the 'other daughter's' diagnosis?

Is your mother of sound enough mind to handle things in a business like manner?

Just because this 'other daughter' has a mental illness does not mean she might not have a valid point about the primary caregiver which is who because your profile says you are a long ways away?

The idea that you just might be writing "other daughter" off automatically just because she has a mental illness does not make me very happy.