Deposits in Post Office with Nominee and Legal Guardian is not sa

Can anyone Let me explain what is the Nominee and Legal / Natural Guardian Clause stands in Post Office Savings Bank Accounts and MIS Deposits for Minor. As my sister Deposited Amount in her 2 Minor Daughters Accounts for their education purpose and his husband as the Legal / Natural Guardian and interest Amount which coming from the deposit is going to her husband savings bank accounts where his wife (my sister) is Nominee. Now all of a sudden or my sister and her daughters bad luck, her husband has been passed away, during this time the Original Pass book has been misplaced and even asked her in laws regarding this and they have said it not been taken. The Police Complaint has been lodged and along with all documents she has claimed at the post office for settlement and change in Legal / Natural Guardianship for her daughters Deposits and asked for the duplicate pass book. Now her in laws came up with the original pass books and served the lawyer notice to the post office to stop the claim, based on this the Post office has stopped the Claim and other process and informed me even they can’t issue the duplicate passbooks since the original pass book has been identified. Can you please let me know, in what basis the post office has been hold the claim, even the post office is not responding for the same. When there is Nominee and Guardianship is mentioned what is the use of these Clause. That means Deposits in Post Office with Nominee and Legal Guardian is not safe, the Grand Parents or others can come up the original passbooks and issue lawyer notice to stop the claim. According to me Blood Relation is different and Relationship with this particular accounts is different, if Postal Departments is interested in family dispute issues let them settle all the family issues raised to her. I think we want to create the public awareness, where there are lakhs of Deposits done in their children’s name for one or the other reasons or planning to deposit in their children names, but this can create problem by known /unknown persons by submitting the original pass books. By providing the business to postal department, the customer can be a victim for these.