Land despute

My father purchased land in year 2018 and same time registry was done. In March 2020 my father started home construction there and some of people came and said this is my land. Even they had also shown registry which was legal and registry was done in 2016. Now we came to know that property dealer is died in accident and even his wife and daughter they are saying we cannot do anything. We spoke to the landlord about this who had sign at the time of registry and he saying to wait and not to file case against them. After checking in land record we came to know, land under the name of his grandfather and not divide between there brothers so we cannot get approval for home loan. 1.what is the legal procedure for this. 2.can we file case against his wife and family? 3.How we can get our money. 4. What is the best way to deal with this situation. 5.My father is very innocent man and he easily trust any one.. Still he is waiting as it was advised by landlord to wait till the solution come out.