Mother did not divided property equally, please advise what to do ?

Respected Sir/Mam I have seen property registered documents where that shows as if mother devided the property (Cofee estate of total 50 acrez)among two sons(elder and the last but one), though family had one more son and two sisters. Excluded son got inter-caste marriage, so as elder son only earned the whole property and he was under government service, he took decision to make a registered documents just to show as if the whole property csme from mother. That elder son is my father-in-law. And his brother sold his share of thd property to some third party from same town.But the house where my in-laws stays and surrounding total 2acres,which was belonging to his brothers survey.During registration of the property sold to third party, there was a mutual talks to exchange land about total 2 acres where my father-in-law's house exists with that of the other 2 acres land to the new party. Now my father-in-law brother and the new party threatens my in-laws to vacate,how can we file?