Can Relinquishment Deed Be Revoked.

My mother relinquished her share in my Nanaji's property on being emotionally blackmailed by my mamaji that he has just undergone a heart surgery and doesn't know how long will he live and wants to get the the title of the property transferred in his name so that he can execute a will to distribute the rights among his daughters later. The relinquishment deed was registered in front of a Delhi SDM in 2008 and my Nanji and Naniji and Massi were not alive at that time. I was in England on a office project and my father was bed-ridden due to a brain stroke so my mother had no -one to consult. She regrets that decision now as my mamaji has cut-off all cordial relationships with my mother since then and she feels cheated. Can this deed be revoked by any means as per law.