Purchase of land from muslim woman using power of attorney

My Father purchased a piece of land from a woman in 1995 in Madhupur (Deoghar), Jharkhand. The Land was part of 20 Katha plot, which got transferred to muslim women after the death of her husband (Inherited Property). Muslim women sold the land in pieces using power of attorney. Muslim women had 10 Children (8 Sons and 2 Daughters), in which 6 of them were adult (above 18 years of age). The muslim women sold the land, and in the registry paper two of her oldest son signed. After 20 years, when I went there to build home, they have refused me from building home and demanded ransom. They alleged that, the Registry paper and power of attorney didnot have sign from other adult children, hence the registry becomes void. They demanded money to give their assent. When I rejected their offer and complained to the police regarding their action, they filed title suit for the same. Please assist in this regard. Are their claim valid or it's a propaganda to extort money? Should I compromise with them or there is any legal help available to me?