Can ex-husband claim property if registered on wife's name

sir, my sister got divorced 3 years ago by mutual consents. now her ex husband asking for the property to give him back which he bought in past but the property is on the my sister's name. He is also threating us by saying that if we will not give him back he will come and insult us in front of society and he is also saying that he will forcefully enter in our house and vacant it in one month by any means. please suggest what we can do and Can he claim on this house (registry is on my sister's name) ? my sister and her ex husband now both are married to different people