Paternal Grand Father biased to share his property with my family

Religion (Hindu) My Paternal Grand Father was biased to share his property with our family. He had 5 children (3 Sons& 2 Daughters). My father was second born among all and elder son to grandpa. Grandpa promised to share 6 acres out of 18 acres during the marriage. Later he changed his voice showing biasedness for 25 years. Grandpa shared his property with other children but refusing to share with our family. He ignored us. Step Fathers are enjoying the property still eagering to have the left out property. What could I do to get my rightful share? I'm elder son to my father and elder grand child to grandpa. Both grand parents are alive. Suffering for justice. What are my rights to win our share? Show some light, not having enough to pursue further studies. I'm grateful to your answers and solutions. Thanks in advance.