Is my father eligible for his aunt's inheritence?

My father's aunt (paternal grandfather's elder sister). Brief Intro, She was childless, her husband died in and around 1994, converted into christianity longtime ago. She got partial mental disorder around 1997 later moved into our family. We take care of her since then and wrote a will to my father. She brought an opponent grandson (younger sister's grand child) due to ego on my paternal grandfather& grandmother acts. Grandson's family obsessed with the property. They made her into mentally sick person. She was murdered by them in late 1990's as she was refused not to share her property in wrong hands. Since then my father and opponent(grandson) are fighting over the rights in court. Currently land is in our position. We are depositing annual two term deposits of income earned through agri land. What are my father's right to win the disputed case? Fighting for justice since 1998? Not financially strong as the opponent? What are our chances of winning? Thanks in advance