My family has been cheated out of rightful inheritance through deception and lies.

My maternal grandfather has 4 sons and 2 daughters, one of them being my mother. My grandfather had two huge plots(let's call it L1 and L2) of land to his name. Each being around 2-3 acres. One plot of land L1 has been divided and given to each of his four sons for building their family home. Except for the third son all the other 3 sons have their family home built on the land. The third son has built a family home in a posh gated society and built 34 houses for rent on L1 land. The L2 land on the other hand was given for Joint venture with builders in 2011. And the property appreciated massively and all 4 sons got more than 2 dozen flats. All the 4 sons have become crorepatis. I was in my teens and in 2004 my mother was deceived to sign on papers which took away her rights from her rightful share of the properties from L1 and L2. Now nearly all the flats except for a few which were given by the builder to 4 sons are sold and they have invested in other properties. My mother did not get a cent from either L1 or L2. My mother never pursued any action against her brothers. She does not know her rights and thinks since she has signed on the papers it is all but over. My grand father was alive in 2004 when she signed on the papers. Now he is no more. And my mother's sister who is my aunt has also signed on the papers without getting anything in return. What are our options in 2023? Do we have a case since this happened almost 18 years ago? I cannot prove charges of deception and lies unfortunately other than the words of my mother. Can we make a case atleast for the few remaining flats which are still unsold? or can we get small amounts of compensation? Your valuable advice is highly desired. Thank you.