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To stop her from continuing to do this, you need to go to court and file a petition for guardianship
/conservatorship for your father. It would be helpful if he signed agreeing to the court ordered guardianship, but if he does not, then you would have to use his mental health disorders and his medical issues and medical testimony in court to have the court determine he is not capable right now of managing his own affairs. Once you obtain guardianship/conservatorship over him, then no matter what mom does she cannot get him to sign anything because that power would be removed from your father and placed in you as guardian/conservator over him.
Unfortunately, if she keeps this up, that is what you have to do and once you get it, because of her comments about wanting him to die, you could also get an order of protection
against her to keep her away from your father if you choose to do so.
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