Partition issue-took illegal possession

Our family property is lying in single land single survey number and it was divided into three plots. one is for my father, one is for my grandfather and one is for my grandfather's father. above three persons are the first owners of the said plots. my father has one sister. My father's sister got married in the year 1973. In 1988 my grandfather's father died. So his property automatically goes to my grandfather. In 2010 my grandfather also died leaving behind my father, my aunt and my grandfather as his legal heirs. Before his death my grandfather also executed a will in the year 2009 infavour of my father that my grandfather's plot will go for my father. Previously In the year 2000 my aunt got money from my father and executed a unregistered release deed that she release all her legal rights in the family property infront of two witnessess who are also signed in that unregistered release deed. After that my father and my grandmother only have the shares in my grandfather's property and grandfather's father property. Now my father's sister threatening & asking my father to give share in the family property by taking possession of our family property in illegal ways with the support of her son's rowdyism act. And also she told my father that unregistered release deed which is executed by her is not valid in the court of law, so nobody can question her regarding the amount which she got from my father for her share. So now she told my father that she didn't get any amount from my father and also she didn't execute any release deed. She acts like this illegally because that release deed is not registered. Now my father was totally upset. She also took possession of my father's property along with our grandfathers property in illegal rowdyism ways with the support of her rowdy son. So kindly advice and help us in this matter to help my father to get his property back.