Cancellation of settlement deed after father

Dear Team,

Thanks for a splendid service. Here is my question

My grandfather did a settlement deed of this property to my father 4 years before. The settlement deed was registered. The property was my grandfather's inherited one. In the settlement deed grandfather mentioned that he has rights to enjoy the property till his death at the same time absolute right is given to my father from then on. Grandfather also mentioned that my father could change the property ownership (patta). My father felt that he would change the name after grandfather's life till that time let it be in my grandpa's name. My father was paying tax for that property and nicely took care of my grandpa. Before doing settlement deed my father has given required money to his sisters as share as per my grandpa's advice. There is a stamp document which has signature's of his sisters as acceptance of money. That document was not registered.

Unfortunately my father passed away around a year before due to sudden ill health. Within a months of his death my father's few sister's and relatives brainwashed my grandpa and cancelled the settlement deed.  (One sister is against that as she felt this is kind of breach of trust as they got some money). The cancellation deed says "my father is not obeying the conditions" and that was mentioned in present by hiding the death of my father. After that those relatives got some parts of the property as individual settlement deed by grandpa and registered. They also got property ownership moved in their name. We all came to know about this just few months before and shocked and feeling much dejected. (The cancellation is done after 3 years period of original settlement deed).When we talk to some lawyers we get answers that this against law.

1. - Can register office cancel settlement deed  without court order?
2. - How can they cancel without the consent of us who are the heir of our father?
3. - When we asked the register office the answer we get is "this is your grandpa property and he can cancel the settlement deed also the name is not transferred to your father". In the settlement deed it is clearly mentioned that "the document will not be revoked or altered in future"  but still in register office they just cancelled it. How this is possible?

Kindly clarify my questions and advise further steps..

Thanks,

Sai