Can a Nominee Execute a Gift Deed in favour of a Nominee

Dear All, The Property under reference was brought by my father way back in 1970 and later he nominated by my mother as a sole nominee to the said property. He passed away on 13th April '1992 and the Society transferred the Flat to my mother as she was the sole nominee of the property. We are 5 siblings (3 Brothers and 2 Sisters) and my Elder Brother of the siblings has passed away in 2016 and my mother has nominated by younger brother as a sole nominee to the said property through normal transfer process. Last year December '2018 she gifted the said property to my younger brother through registered gift deed without taking any consent from us in writing nor have be renounced or relinquished our rights in writing as Legal Heir's. Point here is 1. Can Nominee or Trustee of the property execute a gift deed in favor of a Nominee 2. Based on the Gift Deed submitted the Society has transferred the Flat in my brother's name and made the required changes in the Name Plate and monthly billing. Is this valid. 2. How can the Gift Deed be valid without we renouncing or relinquishing our rights in the said property. 3. One of my younger Sister is staying with my mother and taking care of her for the past 27 years and want's to make a claim in the property for her share of rights, can she do so. 4. What are the procedure's to revoke or challenge this Gift Deed. 5. Can I write to the Society to share the Gift Deed documents for my reference purpose. Kindly advice what would be the best solution to resolve this matter in the best possible manner.