How to claim my Maternal Grandfather's property?

Religion: Hindu My maternal grand father owned a land in Kolkata, he died long ago when my mother was 15yrs old and my other also died in 1979. My mother had three elder brothers. My grandfather died without leaving a will. And after my mother got married her brothers decided to build house on the plot . Now when I consulted a lawyer to file a partition he said that in order to claim right on the property I would need to prove my mother's relation to her father. To do that he asked me to gather my mother's Secondary Exam Certificate but it doesn't exist. So, what should I do now ? Is there any other way to claim my mother's portion of that property? None of my mother's brothers are alive and their sons & daughters are refusing to admit that my mother is a daughter of that house.