Fraduluent Registration Of Immovable Property

My mother had a plot of land in her name (under survey of India co-operative) where my father constructed a house and eventually sold it to a buyer in 2005, who was occupying the property for an assured amount. The amount was paid in cash and cheque to my father in a year time, but, it was during this span some feuds resulted in cancellation of so called sale deed and which prompted we all three bothers and sister to revoke the power of attorney given to our father for such sale of property. The original deed was somehow not found in our house assuming that it was perhaps tricked out or stolen. Therefore, the status of the property remained unregistered. In this process, my mother passed away in 2005, my brother passed away in 2014 and my father passed away on March, 2016 who battled cancer for four months until he expired. The real blow came a couple of days back from today when we found the property already got registered in the name of the buyer where the seller is my father. Interestingly, the date of registration and capturing of photo and finger prints date proves FRAUDULENT means were adopted to execute such sale deed since my father by that time was literally fighting cancer from his death bed. Now, what should be the course of action. Will it mean prosecution if found guilty. As in this procedure Co-operative members, the buyer the lawyer the witness are all involved. Is this a serious crime if proved. And how to get back the property.