First of all, you need to know a bit about what defamation (libel is written defamation, slander is spoken defamation) is. Defamation is a false statement of material fact communicated to a third party that causes quantifiable economic damage. It has to be false (truth is a defense). It has to be a "statement of material fact" (opinions are not actionable). It has to be communicated to third parties, and the statement itself has to cause quantifiable economic damages (some dollar amount that you can directly trace to the statement). If even one of those factors is missing, then there's no defamation case.
So if it's true, even if it uses colorful language, it's not defamation. Even if it's harmful to your reputation, it's not defamation. Even if it is done with "malice", is not defamation if true. If you told her to get out of the house, even if you were the nicest, most wonderful, most apologetic person in the world doing so, it's still putting her out of the house, and if she has no where to go, that leaves her "homeless". Again, colorful language, certainly. But I don't necessarily see how it would be false.
Now you could always threaten them with a "demand letter", demanding that you will take legal action against them for any and all defamatory statements made about you in public. Note that this would pretty much be a bluff, as if they said anything about you that was true, that would not be actionable, no matter how damaging, no matter how malicious. It still might be the best way to prevent them saying anything.
You can also state that you're not filing a case with "Adult Protective Services" (APS) against her (as it appears that there was some financial impropriety). APS is a mandated service provided by local social services districts. It involves intake, investigation and assessment of referrals of abuse, neglect, and financial exploitation of impaired vulnerable adults who live in the community. APS workers develop services plans for eligible clients to remedy physical abuse, sexual abuse, emotional abuse, neglect, financial exploitation, or to address unmet essential needs of adults. http://ocfs.ny.gov/main/psa/
You can make it clear that you can still file such a case, but that you'd rather not. If they feel that they need to bring this out, you might need to file just so that you can protect your father and present your own side of it.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable.
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Thank you, ***** ***** luck to you!