It sounds as if a family member is coercing your mom with Alzheimer's. Using "undue influence" in some states is a violation of the law. In any location, it is
a form of financial elder abuse to take advantage of a person with dementia to manipulate the person into giving away money.
It is important to act promptly to protect your mother. The most important question is whether there is a durable power of attorney appointed. If so, and you are that person, you must immediately take control of mom's checking account and other banking and give her only limited amounts of money to use. You can close the joint account and keep most funds in an new account in your name only. If she is confused, she should not be writing checks or transferring funds anywhere at all. If she has a trust and there is a co-trustee, that person can also put a stop to mom having access to money if mom is determined to be incompetent to make safe financial decisions.
If no one planned ahead and mom does not have a durable power of attorney, it is essential to get her tested by a psychologist or by a neurologist to determine whether she is competent to sign a durable power of attorney. If she is, I recommend that this be done right away and that you be appointed as the agent on that document. You would then have the right to stop her from taking money out of the bank. If you can persuade mom to let you pay the bills and help her, that will be the "excuse" for being appointed as her agent.
Now for the worst case scenario. If mom has no durable power of attorney and is too confused to be competent to sign a durable power of attorney document, what alternatives do you have? First, you can report the financial elder abuse to the adult protective services (social services/law enforcement) in your county. Yes, you do have to name names, dates and where and how the abuse occurred. Some counties are better than others at enforcing the law against elder abuse, so don't expect an immediate and perfect response. Law enforcement begins with an investigation. Your mom's competency to make financial decisions is an issue. That is why testing and a medical examination to document her lack of competency is also very important.
In the meantime, I suggest a family meeting. Gather every person together who is related and who is aware of the abuse. Discuss what to do and how to stop the abuser from getting the rest of mom's money. Confronting the abuser can help, as she at least knows someone is watching and the law may get involved. You may want to have an attorney, social worker or other advisor help you with the family meeting.
I also recommend that you report the apparent abuse, in writing to the bank. You can let the bank manager, and operations officers know that your mother has dementia and is being manipulated into taking money out to give to someone who is financially abusing her. Laws vary from state to state, but it is possible, that the bank may be required to report suspected abuse to law enforcement also.
Finally, as a last resort of all, it is possible to ask the court to appoint a guardian for your mom. This is a rather drastic step, and an expensive one, but it is often the only way to stop a vulnerable elder who can't make safe money decisions from having all her funds depleted by a greedy relative. Seek the advice of an experienced elder law attorney in your area. Find out what is involved and if the attorney has any other suggestions as to how to put a stop to the manipulation short of a guardianship.