Property distribution dispute

Our father had purchased a flat in Shastri Nagar, Goregaon, Mumbai measuring 472 Sq Ft in MHADA Colony. We were staying with our father, mother, elder son, second son and daughter. Our father died in May 1995. After his death the society had told us to transfer the flat in our mother’s name. Accordingly, we three children had given the NOC to the society to transfer the said flat in our mother’s name. The society had transferred the same on our mother’s name. In May 2001, our mother died. She had given nomination to the society serially as, 1. Elder Age 57 33.33% 2. Second son Age 52 33.33% 3. Daughter Age 50 33.33% Ie. Equally to all (The ages are as on date). After our mother’s death the cash and the gold was distributed amongst ourselves equally. We had suggested to sell the flat and distribute the amount amongst ourselves. As on 2001, the market price of the flat was 11 lakhs 50 thousand but we had decided if anyone amongst us is purchasing the flat then it’s cost will be considered as 7 lakhs 50 thousand. This 7 lakh 50 thousand would be distributed among three of us. As our brother ie the second son was staying there he had approached us and said that he will make the payment within 6 months and also said if he couldn’t pay us then the said agreement will be null and void. Even after a year he couldn’t manage the amount and give to us for his own reasons. After that we were holding meetings every year about 4 to 6 times regarding the sale of the flat. But he was not willing to vacant the flat and sell it. We had multiple meetings each year till date but no conclusion has arrived. Further, our society had given multiple reminders to transfer the said flat. We had applied for the transfer form to the society according to the serial numbers of the nomination but the second son has refused to sign on said transfer form for his own reasons. We (Elder son and daughter) have given an option to him to give our land share to make the partition of the said flat of 157 Sq Ft approximately each. But he has refused to make the small partition of the same. As on date, the market value of the said flat is 120 lakhs. The society has approached their lawyer and he has given us the intimation that we have not transferred the flat and we three will be considered as the trustees. When the building will go on redevelopment process and the society takes charge of the said flat the keys from whom (second son) taken will be given back to the person who has given the keys (possession) of the flat. We are eager to transfer the said flat in our name and sell the flat and divide the one third shares among ourselves. Kindly give us advice to fetch our shares as he is not in a position to purchase nor to vacant it. We (Elder son and daughter), both of us are willing to give his one third share in cash for that too he is not agreed. Also advise us how to approach the society and transfer the flat serially as per nomination. If we want to go to court which court should we apply?