Property Dispute

My grandfather had foursons and two daughters and was the owner of the property in dispute. He died without making a will.The eldest son separated when my grandfather was alive and took hi share. In 2001 the other three sons mutually separated by signing on a 100 RS stamp paper which was never registered. As per this discussion, my grandfathers property consisting of built house and garden plot was divided among three sons. My father and his elder brother divided house among themselves and the eldest took the open plot. From 2001,my father had paid tax for his share and the open plot as well. Three years back, my father applied for registering his share and he presented all the tax documents.the municipality has registered the "Half house"+"Open Plot" in my fathers name, As of today my dad has gifted the plot and the house to my sister and the half house and plot is in her name. Today by uncle is planning to put forgery case on my father. What are our chances to fight the case.