I'm sorry to hear about this. Best bet is to report the abuse to FB, as a lawsuit would likely cost thousands more than would be recovered. Most defamation lawyers want a $5,000 retainer just to start work. Good luck.
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On the one hand, accusing someone of murder is a statement that can serve as a basis for a defamation action. On the otherhand, the "truth" is always a defense to any claim of defamation, even if it causes the subject of the statement economical and/or emotional harm. The point is that if you commence an action do you want to litigate your father's medical condition in the legal forum, to determine in a lawsuit whether she has a "defense". This is so even if you haven't bee charged with the act accused of. If you are willing to go through that, then consult an attorney who is experienced in defamation actions, in your area. You should follow the advice already posted, that being to lodge a complaint with Face Book. You may also want to consider having an attorney send your mother a "cease and desist" letter - the threat of being sued may be enough to get your mother to stop.
You can seek a restraining order to prevent her from harassing you.
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In Pennsylvania, accusing a person of a crime is defamation "per se." It is an actionable civil claim. You may be able to prevail in a claim, but you have to take a lot of things into consideration. Financially, what do you stand to gain from suing an individual (unless they are very wealthy)? If the funeral director making the defamatory statements actually handled your father's remains, it might be a different story. You might be able to bring a claim against the funeral home as the director is an agent/employee of the business.
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