Everything you have said that has been happening and accelerating tells me that as you are attempting to lock down her assets ~ please contact an Elder Attorney and fight for conservatorship. She can stay where she is and feel/live fairly independent, conservator just has control of her assets, money etc. They will pay her bills, oversee her banking and assets. You could set her up with a bank account where you deposit minimal in each month for her extras with no connection to her other accounts (aka only account at a bank where there is nothing else). There is reporting to be done but she is losing her money way too fast!
I would also start process of filing police reports against her abuser(s) and the banks should be able to trace where the money went.
Also if she owns her home contact the real estate record & tax department and see about adding an alert to her account. If anyone attempts any kind of business with her home you will be notified immediately. We have that on ours just because of all the paper crooks out there. Or you could move it into a trust with her children as trustees. Also at her passing the house would avoid probate.
I am just going to put this out-if mil does not want a conservator then unless she is determined to be incompetent it can't be done. being deemed incompetent is (esp. in these post 'free britney' times) at best one of the most difficult legal endeavors to take on. a 'simple' guardianship (no control of financials) alone that even has the agreement of the individual entails extensive reviews of substantiating documentation from a variety of sources (medical/psych providers to begin with-if you find ones who will agree to it b/c w/o a clear diagnosis they are hesitant and resistant), investigations, interviews and reporting by the guardian ad litem who will represent the individual.
elder financial abuse is horrific (any kind of financial abuse is) but the courts do not want to err on the side of taking away the basic rights of any adult. the process of even a basic guardianship can take months. an elder law attorney can advise if something meets the criteria for an emergency court order but absent existing substantiating documentation and medicals it is likely a long-shot.
all the sons of the mil (and if they all agree to it-their wives) should do a group consult with an elder law attorney. find out what can and canNOT be done. find out safeguards they can put into place with mom's approval. if conservatorship is warranted it can be decided if someone in the family will do it (not for the weak, not for someone who does not want to be the 'bad guy', not for someone who does not want to be responsible for tremendous reporting to/oversight by the courts and potentially adult protective services-many opt for a paid private conservator).
don't mean to talk this option down, just want you to be aware-this is as i'm in the process of doing my son's almost 30 page yearly annual report and plan of action for 24/25/26 that I have to have the courts approve so I can turn around and get his guardianship paperwork renewal paperwork filed (conservators have to file over 2x the paperwork in my jurisdiction).