Son harassing old mother for [property what can be done to stop him

Good day, My mother-in-law is an 84 old Parsi, residing in Mumbai. She has 2 daughters and one son. Daughters are well settled in india and the son is settled in the US. From the time my father-in-law passed away in 2011, her son has been constantly harassing her on his visits to india to transfer the property where she is residing at present to his name. He has made her forcibly made her sign many wills staking his claim on the house and all her movable and immovable assets. Her daughter's have time and again told their brother that they have no interest in the house nor the money but he continues to doubt them and continues his harassment towards my mother in law during his to india. Pls advise line of action to safe guard my mother in law's interest.