Land registered by son in law with informing the father in law

Hi, Recently my Dad wanted to break a registered will due to a disagreement with me and my brother. Will was written to give all land to us after his death. He went and stayed with his son in law for two weeks and he took Dad to register office and broke the will as my dad wanted. Issue is later my Dad was told that the Will breaking had an issue in last name and register is asking him to come again and sign. His son in law used this opportunity and registered all six acres of land in his name without saying my Dad by taking him to register office.No money is transferred during the process. it's mentioned land was transferred as my Dad borrowed money over the year's. Now Dad came to know land is not in his name and told us what has happened. All original documents where with me during this time. Kindly help us. It's not even 30 days yet after this registration. The land was my Dad's earning not ancestral. My Dad's age is 90.