Mother did not give me my share in the property what can I do

A DDA flat booked by my father, was transferred in my mother's name as he had passed away before taking possession of the flat. In 1984.. I was 18. I got married and was told to leave the house due to frequent flights at home in 1998. Later in 2004, I was given around 5.5 lakhs and asked to sign a stamp paper stating that I do not have any rights in the house. I wasn't given a copy of this stamp paper. Now the flat has been sold by my mother and she has given all the money to my sister with whom she is living. Is there anything I can do or anyway I can contest the same. Please advise.