Regarding defamation suite

On 2020, my husband was ill, on knowing that my husband is ill and can't live long my son threatened my husband and forced him and wrote a settlement deed of the properties in my grandsons name. One week later, my husband passed away. 1 1/2 years later my daughter sent a notice to my grandsons about the settlement deed written by a illegal way. So they gave a plot and some amount to settle it off court and I, my son, my daughter, my grandsons all had a MOU written in front of the notary officer. My husband while writing the settlement deed wrote a life estate for the house that I am currently living in. I am staying downstairs and upstairs I have a tenant. My son has threatened the tenant to vacate stating that I can only stay but not earn through tenancy. So wrote a letter to the university that my son is currently working about the illegal activities of my son. Now my son has sent a notice that he is going to file a defamation case against me. But i have no proof regarding my son threatening my husband. I am worried about that. What should I do