Getting sucession certificate

My father in law expired in May 2016 and he is a teacher. My mother-in-law is second wife to him. she had married him some 30 years back. He has quoted in his pension book my mother-in-law particulars and her photo only. For his first wife there are two sons and for my mother-in-law two daughters and all are married. First wife expired in 2011. When she applied for legal heir certificate for the purpose of family pension, tahsildar of Gudiyattam, Vellore Dist, TN has given a letter stating that there are two wife and settle at court of law. In legal heir certificate she has mentioned herself and sons and daughters. We approached a lawyer at Gudiyattam, he said a affidavit to filed in the Hon'ble court. Now two sons of first wife are saying that they don't sign the affidavit which is to be filed in the Hon'ble court at Gudiyattam. What we should do ? Is it necessary the signature of sons and daughters in the petition to be filed in the Hon'ble court. Kindly advise. My mother-in=law does not have any source of income other than the pension.