Transfer of property from deceased husband to wife

Hi, My father purchased the flat in Vashi in 1990. He was the first owner of the house, purchased from builder. It's not a case of reselling. He happened to miss out to fill in the nominee form during closing the documentation part for flat. We are living in this flat from last 15 years now. Years passed and he forgot to update the name of nominee in this property. Last year, 30th Sept, he expired. We are three siblings and my Mother who are the legal heirs of this property now. However, both my sisters are married in well settled family. When I approached our society office, I got to know that there is no nominee mentioned in the form filled by my father. So, society gave me the bunch of papers and asked me to get the required documents to transfer the flat to my mother's name. As told, I submitted affidavit, indemnity, name change request form of society, declaration of no objection for transferring the ownership of flat to my mothers name of all three siblings and new nominee form to CIDCO office requesting them to change the ownership. As per the process, I paid 5000 rupees fees to CIDCO for name change and in 15 days, CIDCO changed my father's name to my mother's name and issued the letter to our society mentioning that, we have changed the ownership of the flat in our records, and kindly change the name in your records issuing the new share certificate on deceased wife's name (i.e my mother). I submitted those papers and the CIDCO notice to our society, where I was told that all the required documents are on place and we will issue the share certificate on your mother's name in 15 to 20 days. However It's been an year now, and now society is demanding for heirship certificate for which I need to spend around 200000 rupees and wait for another one year to meet the required formalities. My Query is, why is heirship certificate required when my mother is willing to sign the paper mentioning that any new heir claims this property as my father's wife, I will be completely responsible. Also, don't we gave any provision that if husband dies, bydefault owner is wife for the property. Society is not willing to transfer the flat and now giving daily new reasons and new document requirement. To your surprise, recently I was told to submit the letter issued by bank where my father had account and he has mentioned my mother as nominee. What is the actual procedure and what are the required documents for this process Please revert on my email address galajits@gmail.com or you can also contact me on +919769144905 Thanks Jitesh Gala