Acquiring property illegally

6 years ago

My grandfather bought a bungalow in 90s. He has three son and one daughter. As the younger son was in central government service, the property was registered on his name. Bungalow had total 9 rooms like three separate flats. Ground floor has 3 rooms, 1st floor 3 rooms and on backside ground floor 2 rooms 1st floor 1 room. middle son lives on first floor, younger son lives on backside, and my grandparents lives on the ground floor, which technically belongs to eldest son, my father. After some years property was transferred to my grandfather's name. Whilst middle son separated his flat on first floor with fabricated door and grills. My father, the eldest brother of them, left the bungalow 10-12 years ago. But we still have a good relationship with my grandparents. 1 n half year ago, middle brother asked for 1 room on ground floor which belongs to us, to practice law for 6 months. one year ago younger brother built 1 room extra to his flat.
Now the middle one's son is going to get married. He is asking permission to my grandparents for building an extra room on roof. While we asked him to clear ground floor's room first as he asked it for only 6 months, and now it's been 1 n half year, he said that he won't clear the room. My father, the eldest son, never interfered in these matters before. But now he wants to claim the part of property that belongs to him. As the middle son is not leaving the room peacefully, can we clear it forcefully?
What should we do? Please help.