What to do if their is a land dispute

Hello sir/madam, our ancestral property was in the name of my grandfather, which before his demise was not given by him to my father but automatically passed to all siblings (my father and 5 sisters) equally. My father along with mother and me and my brother used to live in Jharkhand due to my father's job. My father registered the land to his name in 2008 from the sisters who gave their consent at that time. After the land come to my father's name he built a house in that land and after his retirement we decided to settle here. Previously, three of the five sisters used to live here(one married with her husband and two unmarried sisters). After we shifted here, they started to create problem for us in innumerable ways and they are now asking their share of the land back. Can they ask for their share now? What steps should my father take to prevent this from happening? My father has been providing them household money for the past 50 years.