Alimony dispute for aged couple

My mother in law sold her own ancestors property and brought a new property and she registered the property on her and her husband's name.They are separated as father in law was drunked and used to harass physically and mentally a lot. They are now separated for 3 yrs because he had started mentally harassing his son who is 25. He had borrowed lot of money from people and everything was settled with selling of mother in law's properties.he left not even a single rupee with mother and son and Now when he have lost his job and has no income he wants his 50 percent property to stay or as value of cash .in whole life whatever he made has been wasted on drinks and cards.son is now 28 year old but he has no income or job. My father has given him some money to start a business but nothing has been yet started hence now both the family expenses are taken care by my dad.Hence this jointly registered property is the only property available with us now.how can we save the property from father in law.