What to do if my father has been threaten for property

Hi, My uncle and his two sons are life threatening my father for property. My father was an adopted son to his own uncle. Now my uncle is saying he won't give the property of my father's biological father. Till this time the property of both the fathers was being shared. He is staying at the house which belongs to my father's adopted father. He has also taken half of the land which belongs to my father's adopted father. Now he's giving life threats to my father, saying he won't let him take anything from any of the two properties. What should I as a daughter do to help my father? What should he do to come out of this and lead a peaceful life?