Succession certificate

Sir, My husband was in Navy. I am a housewife and have a 3 years old daughter. My husband passed away on 6th may due to COVID. NOW, it came to my knowledge that he had made his mother and father as a nominee in his bank accounts. His father was a govt employee and getting pension plus his retirement money,mother a housewife and they have a home in patna in name of my husband's grand father. Both Parent in maw have fixed deposits as well.My husband had bought a flat in Pune , which is in my name.however, loan amount is not yet paid fully.Only in PPF and in life insuarance my name is registered as nominee. However, husband had shared his net banking details and atm with me for one account. All his property and other documents are with his mother and she is denying to give me any papers. I have sent mail and legal notice to bank regarding block his bank operations until I get death certificate, which is yet not been issued. Bank is not at all co-operating though.Sir, pls help what can be done in this scenario.. I am helpless and broken.