Previous owner of land is trying to claim position after legally registering the land 24 years back

Hi, My father and other person purchased agriculture land from a owner in 1989, In 1993 other person sold land to my father and the registration is done in sub register office. My father also have got the sale letter written ( one of my relative wrote as my father did not study ) in that doc it's clearly mentioned she wants to sell her land and we have taken signatures of her son and husband. The original register doc's are with us. Last week she has sent a lawyer notice stating that she is still in position and while they were doing agriculture we went and threaten them to vacate the land and warned them to create fack documents and sell the land. I approached local advacate and asked for replying to the notice, he is saying that , as we have no 'cash receipts" she will file the case and she can win, and she can say that the document we have were either fack or were forced to be signed by her. We also have witness on the registration doc's and the sale papers, who are alive. Please suggest what should be done in this case, as It's bothering my family Thanks