Property dispute

My father ( 73 years) & brother ( 45 years) jointly own a flat in Mumbai. Part payment was done by my father and brother took a loan for the balance amount . Does the amount of payment made by each party has any significance on ownership. The agreement does not state anything to this effect. The loan has now been repaid by brother. We are 2 sisters-myself ( 46 years ) unmarried & my sister ( 42 years) is married. My brother stays in the said flat with wife & children and my dad stays with me in a different flat which is owned by me. My mom has expired. Below are some of my queries: 1. My father wants to sell his share but my brother is not willing to sell. Nor he is ready to buy father's share at market price. What recourse my father can take? 2. My father has not made any Will. In such a scenario how do I & my sister protect our share in the said flat. What steps we need to take to ensure our share is protected now and even after my father's death. 3. As it is a jointly held flat, what happens in the event of my father's death ? Does the ownership of my father's share get automatically transferred to my brother or he has to go through succession process whereby he would need our ( mine and my sister) permission to transfer my father's share to his name? Can succession certificate be requested by any legal heirs? 4. My father has nominated myself as the beneficiary of his share and this nomination is registered with housing society. Does it benefit me in anyway? Me and my sister are keen to get our share in the flat even if we have to fight legal case against our brother. Looking forward to receive your advice at your soonest. Appreciate your assistance in advance.