Post Office Deposit claim by Nominee after Holder deceased

Post office made minor daughter of my sister-in-law as a joint holder of post office account along with my mother-in-law without any KYC documents in Yr 2015. In that account, nominee is my wife. Now my mother-in-law has died, and we dont have the passbook but a police diary which was filed accusing my sister-in-law of stealing the book from mother-in-law before her death, even she has withdrawn the monthly interest using minor holder's signature. On basis of police diary, we got access to the account information & submitted claim after complaining the matter to the ASP Divisional office of post. The post office is in big time fraud & is now delaying the claim process. Almost 25 days over. Who will get the money? The minor holder or the nominee? Is there any way the parents of minor can challenge the nominee claim?