In some cases like this you have to take the unpleasant step of starting a guardianship.
Mother is in danger and doesn't realize it or fails to acknowledge it.
The only way for you have control over the caregivers is for her to trust you with a power of attorney or guardianship proceeding.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
There are specific statutes regaring caregivers and being added to wills. You need to summarize your facts and consult a wills/trusts/estates litigator as soon as possible.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I agree with Attorney Pippen. If your mother is incapacitated, and it certainly sounds like she is, then normally a general durable power of attorney would be called for. You do not mention if your mother has one, but it seems likely that if she does, it names her caregiver. You have probably waited a bit too long to take action, but I believe you may need to petition to be appointed your mother's guardian and/or conservator. The potential problem I see is that the Will apparently reflects both your mother's incapacity and the undue influence of her caregiver. You are going to need to ask that the Will be set aside, at some point. Because you have evidence, now, it may be necessary to take that step before the caregiver walks away with the entire estate.
You are going to need a lawyer to assist you with both matters. There are many fine estate planning and probate attorneys in your area that are very active participants in the Avvo community. You need to contact one, right away.
Best of luck to you and your mother! She is counting on you to protect her, at this point.
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
I suggest you contact Adult Protective Services for San Diego County as soon as possible: http://www.sdcounty.ca.gov/hhsa/programs/ais/adult_protective_services/index.html
If your mother lives in a different county, then contact APS there. It is quite possible your mother's caregiver has committed financial abuse, but the time to determine that is now - not later.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the State of California. Responses are based solely on California law unless stated otherwise.