Ancestral property dispute between children of Two wifes

Dear experts, My father got separated (without divorce) due to some disputes from an women and two children before 30 years and married my mother. That time he doesn't have even signle property / rupee on his name. Since then he was staying with us and he earned some properties (all are self acquired). We have all the documents to prove ours as a family. The documents such as Ration Card, Aadhar Card, Voter card etc. were with our address. My mother also there in that ration card as his wife. We even enjoying the benefits of all the properties earned by our father. A month before my father was deceased with heart problem. We obtained Death certificate and even Family member certificate from competent authorities. Post my father's death during the sale of our grandfather's property we gave share to the son of first wife's. Then he even signed on a 50 Rupees stamp paper by promising that they won't make any further claims on my father's properties. Now we heard that they are planning to file a civil suit for getting share in our properties. Hence, The question is 1. Whether they (my father's first wife & children) have any right to claim share from our properties? 2. Whether the stamp paper signed by first wife's son by promising no claim will useful for us? 3. What we supposed to do in this situation. Please provide any useful suggestions. Thanks in advance.