Get Back The Mortgage Property Papers

In October, 2018 My father, mother and myself jointly purchased a 2bhk flat against a home loan from Axis Bank means we three are the property owner without mentioning any specific percentage of ownership. That loan was issued against my fathers name as the main applicant and my mother and myself as the co applicant......on February 2019 after my fathers sudden death the bank emis paid from my end in regular basis after few month the bank discuss with me for settle down the loan completely with some concession amount as the loan was not covered under insurance and the papers will be back to us. As per there proposal the bulk amount had been paid and the loan account get closed after 4 months. Afterthat when I visit the bank for the paper release by providing our legal heir certificate (Although my married sister name also included on that legal heir certificate) attested from the first class megistrate of district court , my fathers death certificate, and all the legal heirs kyc....they now also demanding the succession certificate to get the paper release and I think they harass us. My question is now where the out of three two registered owner of that property already survive then is it mandatory to provide the succession certificate to get the original papers back as per the law??? please suggest in detail and help.