Division of FUNDS between fathers two families

My father had two marriages. First wife had 2 sons and one daughter.. one daughter and one son is married. First wife has been died. Then my father married to another girl.. I m the son of second wife. We are three brothers. My father was a govt employee. Now he is no more. . Now the problem is arised about division of PF and other funds which will come.. There is a letter came from govt about Gratuty .. which came by mentioning the name of my mother who is second wife.. now the sons and daughter of first wife saying that they want half share in the amount of gratuity.. Are they should get half share or not.. after the death of my father sons of first wife said that there are No nominee in the service book. They said us to write an affidavit to divide the funds in equal shares.. but now we have found that there is name of my mother (second wife) in the nomination page.. what we should do now