Co op society redevlopment

Sir / madam, My late father had bought a flat in Mumbai in 1980 on my mother's name where first nominee was my father and second was my name but in 1987 my sister who got married in 1982 and settled in Singapore took my parents in confidence in 1987 and changed first nominee as herself and second as my father. Since 2000 me and my wife and two children's were staying in the said flat but in the same year due toy profession I had to shift out state. In 2003 my mother demised she did not prepare any will not it is been produced by my sister till date but still in 2005 without letting me know or without my concent she signed the deal and agreement with the builder for the redevelopment of our society also took all te corpus and advance rent money and after completion took possession of the redeveloped flat and has rented out the said flat of which rent Is also been taken by her and due to all this I have to stay withy family in rented flat for so many years please suggest me what should I do