Unlawful possession

I have 2 room house (30+ year old) built on land which my Late. grandfather had purchased from two people(both alive) let's name A & B and this was a mutual understanding hence my grand father didn't do any paperwork. There was some situations created because of which my father had given key of 1 of the room to my Patidar( son of my Late grandfather's brother) in 1997 to live because he had no child. And after that my father manage to bring A & B to court and got their consent for this land transaction and notarized it in 2001. In 2006 my patidar sold his stake to person C and given key of that room also & this transaction is on stamppaper only with witness signature, without our knowledge and died. And since last 8 -10 years person C started keeping paddy in that room and they never locked it. Now since Feb 2021 they started locking that room and when my father objects it then they called cops and shown that stamppaper. Now we also have putted our lock so person C also can't use that room. Can you plz let me know any work around to get my room back as it was purchased by my late grandfather and hence his brothers can't claim their stake in it. Then how my patidar can sell that and how we can get our room back.