Regarding dhana settlement deed

I have 2 sisters and im the elder brother. My mother wrote a dhana settlement deed on my only son. I did not transfer the deed on my son's name as he was a minor that time. After 8 years mom fell sick and not mentally sound. My sisters took advantage of it and took my mom to register office and took her thumb print, as she is illeterate she always use thumb print. Later they cancelled the dhana settlement deedand registered the land on their name.after 6 months my mom died. I came to know about the cheating of my sisters after my mother's death only. We gave allthe remaining properties to my sisters and they got huge sum of cash too, despite that they betrayed us.Right now case is going on, my sisters are not appearing at all, they are keep on getting vaida mentioning 1 sister is out of state, 1 of my sister lives in Maharashtra with family. 1. Is it possible to cancel a gift deed after 8 years? 2. How can they register without notifying me? Does the law allows that? 3. What are the ways to get our property back? 4. Is there anyway to prove that my mom was mentally ill and they cheated?, Please , we don't have proper guidence. We seek your advice as we are in a lot of trouble. Kindly help us.