A cease and desist letter might help but proving slander is no easy task particularly in intimate family situations. Consult a defamation attorney for the cease and desist letter and further clarity. best of luck.
The author of this answer is an Attorney-at-Law, licensed to practice law only in the state of Arizona. Unless both you and the author have signed a formal retainer agreement, you are not the author's client, and the author's discussion of issues does not constitute legal advice. Opinions expressed herein are solely those of the author, and are neither privileged nor confidential.
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