cheatng in making papers

My brother took loan of 20 lakhs from an individual financer @ 2.5%p.m. interest by mortgaging ancestral property worth above one crore; acquired through registered will. THough RD is not yet given by any sister in his name. The financer instead of making mortgage deed has made and got signed GPA, agreement to sell, possession letter and will in his name. And the copies of these agreements were not provided when these were executed but have been given after about 1 and half year on repeated requests. He has been paid interest for 13-14 months in cash for which he has not given any receipt and also 6 lakh principle amount has been transferred through NEFT in his account before getting the copies of all the agreements as above. Still we have doubt that he has made some other agreements and got signed from my brother without disclosing the actual contents of those papers and copies have not been provided to him. These agreements are got registered also in the records of concerned authority. In agreement to sell it is stated that my brother is an exclusive owner of the said property though RD in his favour is not signed. Now the financer is asking to sign RD in his name. Can we challenge the sale agreement on this ground tgat my brother is not an exclusive owner and the financer has cheated him by making wrong papers and got them signed by deceiving the actual contents of the agreements. What legal remedy is there for us to save our property and what counter action the financer can take if we make complaint against him for cheating. How can we prove the cheating though I have call recording in which it is discussed that we have mortgaged property but he has made some other agreements but not of mortgage. Now the statements coming from the financer side is that my brother and bhabi as witness knowingly signed all the papers though this is not correct. He did not even read the papers for the reasons (1) he is less educated to read and understand the documents in technical english (2) the mediator of this mortgage deal is having business relation with financer and is son in law of our cousin brother who is like our real brother for us hence trusting him completely, my brother signed all the papers who on the receipt showed only the amount to be given to my brother through cheque and cash but not other papers and their contents. Plz suggest what all we can do to come out from this trap and save our ancestral property.