Wife claiming her rights after husband's death if she is not a nominee

My brother in law expired and he has his wife and 3 year old son. In his Office, he has nominated his mother for all fund even after marriage, he updated the nomination and again nominated his mother. Mother is widow and having two other sons. Brother in law showed his mother as dependent on him in office medical cards. Now after his death, my sister filed claim for funds in office and her mother in law also. Mother in law is not ready to give single penny to my sister and her 3 year old son. My sister is housewife. So she has no source of earning. Now office demanding succession certificate for the same. So I want to know how we can apply for the same, if other party doesn't agree?