Claiming Legal heirs of Ancestral property -- forceful occupancy thereof.

My father came in India in 1948 at the time of partition from East Pakistan Now Bangladesh. A piece of land was donated and registered by RR Department of W.B Government in my father's name under refugee rehabilitation scheam. in the year1999. Now this land was developed and made durable for dwelings by me with huge expenses provided. After death of my father , the building was assessed by KMC of West Bengal and subsequently mutation was done against Four brothers including me. The building was not partitioned and was dweling as joint family with seperate daily amenities for each family. Now a married daughter of my younger brother forcefully occupied some portion of the building and claiming right of ancestral property of her grandfather though her father is alive and also spend a big amount of money for errection and develop the building as a part. Now my humble question is whether this forceful occupancy was legal when her father is alive and denied to do so? If it is illegal then what action should be initiated ? please advise. ---- Manik Dey, Kolkata 700078, West Bengal, India