Mortgaged property of mother

Hi I am the only son of my parents ( father died few years ago ) They mortgaged the residential property ( reversed mortgaged ) that they were living in * the bank took our signatures ( i am not sure on what papers but we are sure that it was not on the guarantee paper, For few years they get Rs 35000 monthly and sometimes they look lump-some amount,,, in my knowledge my mother must have taken over 50 lacs from the bank ( none of the money is spent on my family or me ) she spends on GOD, Temples and bla bla bla, resulting her hands are empty until she gets new installments. its been ten years , she also gets father's pension around 6000/- Since it is the only house, i am always worried that bank will claim this amount from us and will action it in case we cannot repay it. She always think that my father gifted me Rs 60 lacs from his earnings which is a totally white lie. I have 2 sisters who are married and billionaire ...and has good terms with me and my family but 3rd sister has spoiled relations with everyone. ( not with me as we are not talking to each other just because my mom doesnt like her at all ) some times my mom warm me to get out of house by showing anger on domestic things...sometimes she says that this property belongs to me . I am in a fix , in case i repay the bank...all three sisters can ask for the share in case of absence of will if i dont pay to bank, the bank will action it and distribute the balance amount to four of us... I cannot refurbish the house fearing that the amount spent by me will be distributed to all heirs ( why should i unnecessarily spend 15-20 lacs ) I am sure that two of mu sisters wont claim.,, but what to do with my 3rd sister whose cards are not yet opened ... (The flat was in my mom's name and mortgaged with bank. it was my father.s self made property) Please advice