How can I save my flat?

Hello I am writing on behalf of my mother who is a house wife. In the year of 1986, my mother and her brother in law jointly paid for a flat in Mumbai costing Rs. 4,05,000 (at that stage). My mom paid 40% of that amount in CASH and her brother in law paid the rest by CHEQUE. My mom has NO proof of payment of the flat. The flat is in the name of her brother in law's company which does not exist anymore. Before 2010, my dad claims to have lost the original copy of the flat's agreement. However, nowhere in the flat agreement does it state that my mother is a joint holder of the flat, they have only mentioned HIS NAME in the agreement. My dad, along with his three brothers and parents have always harassed my mother, brother and myself financially and personally. They have a history of repeatedly blackmailing and threatening my mother. The problem is that my dad's brother (who initially paid) is very capable of removing my mother from the flat - he has created a similar scene earlier in 2012. Now, he is planning to sell off the flat without our consent. We are in a deep mess and do not know what to do. What do you suggest? Can he remove us from the flat forcefully and sell it? Is there a way that my mother can get a second name in the flat? Her brother in law and her father in law is not READY to put her second name in the flat. Please help us. Looking forward to hear from you. I appreciate your help. Thanks, Niyati