If there is no nomination for the fixed deposits in the name of your deceased wife in your or son's favour, apply for succession certificate for the same impleading your mother in law, in appropriate court. For all the assets including those in bank lockers in the name of your wife, you and your son or if t here are other sons or daughter, along with themfile a suit for declaration and recovery of the properties of your wife, impleading your mother in law and the concerned Bank. In that suit get an injunction against your mother in law operating the locker, have a court commissioner/Receiver appointed to take custody of the contents of locker, pending the disposal of the suit. Get the assistance of a good local lawyer.