Fraudulant sales deed transfer

My mother stayed in Guwahati,Assam till she died recently after a brain stroke and subsequent seven month staying in hospital under my care in Guwhati. I work in Mumbai. Before her death she made a unregistered self signed "will" with only one witness signature,in the name of me and my sister who stays in USA. Taking help of her old age(aged 74) and staying alone, one tenant staying in the same building for last three years in the same house in ground floor manged fraudulantly to make her signed in three sales deeds in 2020(during COVID time) in July 2020 and October 2020 and last one in July2022 to take the entire three storey building along with 95% of the buiding land.She is unaware of what is written in the sale deed sgreements as she trusts the tenant and the tenant made three sales deed bringing court notary person home without her going to the court in three cases even during the COVID lockdown period. But few days before her death one of the friend of that tenant, told my mother and me (calling me to Mumbai) that the mutation of almost 95% of the land other entire buildig was made back in 2020 by the teanant but kept it silent for two years.After we came to know th matter from the tenant's friend that almost entire land was mutated by the tenant my mother got great shock, as she told me that she was never intended to sell him entire land and the building.only she wanted to sell four rooms of the ground floor of the building as there was an accute water supply probem in that area for last four years. Next after this incident, the tenant made a another last attempt to muuate the remaining 5% person of the land in september,2022.But it was stopped as the tenant's friend informed us immedialtely after the tenant took signature from my mother in the mutation receipt(pretending that the paper is a rent receipt,so no need to read it and mother signed there). The friend sent me the scan copy of the signed mutation receipt immediately. Since one week time is given after signing the last mutation recipt by the Circle officer to confirm any third party's complaint if any, so, on my request on the very next day the circle officer called my mother asked the matter and cancelled the last mutaion attempt made by tenant. Now I am preparing a injunction through a lawyer in Guwhati for injunction along with the cancellation of the sales deed request. Apart from that I have lodged a cheating(420) complaints in local police station after my mother's natureal death few days back. Also mother gave a first FIR in septrember 2022 when she was threatened at several night by the tenant using other third person to call to her mobile number and asking her to vacate the first floor of the house where she stays. Now my question is how I have to approach on the criminal side of the case to make the tenant confessing his fradulant deed if he does not confess to police that he has taken the signaures from my mother several times showing some other papers to sign.