To whom does your mother want the flat go after her death? Is it to you only, your sisters or all of you? You are saying that your mother inherited the flat from your father. Was it by nomination or by will? From whom does the Society want NOC, your mother only or also all her children? How are your sisters staking claim for the block? Have they written letters to the Society? If so, what is the basis of their claim as stated in their letters? Now my opinion given below is subject to your answers to the above questions.
Your mother can very well give NOC to the Society? Her rights cannot be questioned. Her rights are on her interests in the capital and property of the Society. After redevelopment she will have the same rights on the resulting property after the redevelopment subject to the terms of redevelopment. If she dies in the meantime, the Society can safely transfer the property to the nominee, if she had filed nomination. No one can stop the Society from that. Her legal heirs can claim their shares from the nominee. But until the heirs succeed, the property will remain with the nominee only and the Society can directly deal with him without reference to the other claimants, if any.
How many years is it after your father's death? Other than the flat, has she inherited any other property from your father? Is she getting a family pension after your father's death? Are you staying in the same house as your mother ever since your father's death? How are the living expenses of your mother met? Are you meeting the expenses? Have you got record of all amount you have spent on the flat and for the redevelopment? Are you paying the monthly or quarterly maintenance charges to the Society yourself? You should maintain a meticulous account of all these supported by record of bank transfers.
In case your mother wants to give the flat to you only she can write a Will in your favour. In the Will she must give full justification for bequeathing the property to you only. She can mention the expenditures met by you as full or partial justification. If she is not able to understand what is written in the Will, have a neutral well-wisher explain things to her. She should also write that the Will was explained to her by so and so. He or she should sign also as a witness. There should be at least two witnesses. Registration of the Will is not essential. But it is good to register it, if possible.
Everything said and done, if your mother gives your name as her sole nominee, your case is more than half done. It will be an uphill task for anyone else to claim from you.