Tenancy rights of a parent to the children

My mother-in-law has been staying with her in-laws, her husband and her children since 1945 in a rented apartment in Mumbai. She is still alive. She has 6 children (3+3). Now the bldg has come up for redevelopment and a developer has given a draft agreement to be signed for this purpose under which we will be entitled to a flat. Since this is a very old property, we are not sure of existence of documents like rent agreement, etc. We also need to find out whether there is a rent receipt and if so, it is in whose name. I think this may be a crucial factor. Now the problem is that one of her sons (my brother-in-law) seems to intend to grab all the share of this to himself. He is also handling all the matters with the developer. He has just approached my wife for an NOC, but has not given the draft of the same. We are waiting for it. Now my ques: 1. If my brother in law would have rent receipts in his name, can he deprive us of our right, as the children of my mother in law ? What if he does not have rent receipts, but has electricity bill, ration card or water bill i his name and paid them ? Then can he have an advantage over others ? 2. What steps can we take to stop him from doing something wrong and grab all the rights and the sale value or compensation ? 3. Can we get a legally enforceable paper like a bequeath or a will from my mother in law stating that the rights of the rented property accrues to all her children equally ? Will such a document be valid and enforceable ? 4. What are the various legal challenges that may come up and how the rest of the 5 children can protect their interest in the property ? Chellamani