Sale of flat of deceased father, flat jointly owned by father and mother

My parents have jointly taken a flat in Kalyan, Thane, Maharashtra. First owner was father while Second owner is mother. In Society Share Certificate 50% shares are allotted to father and mother respectively. Unfortunately My father died without a Will in February 2017. Also we forgot to give Nomination for his 50% owned flat. Now, My Mother, I and My Younger Brother are the Class 1 Legal Heirs in this Case. . Now, We have decided to Sale this flat Asap because one party has approached us for buying this flat. But, To Transfer 100% Shares in My Mother's Name Society is asking for legal heirship certificate or Succession Certificate from Court. Because Father died without Nomination and Will. . There is no dispute between Three of us to Sale the flat. We both brothers don't want to claim any ownership on the flat. In fact We are ready to make Release deed in favour of our Mother and Register it in Sub registrar office of Kalyan. But, Since we don't own any share in flat Currently We can't do it. Please kindly Advice Me the Correct Procedure to be followed in this Case.