Family property share issue between brothers

Hi Sir Need a clarification regarding family property share dividend. We are three brothers and my father married to two wives both are sisters, first wife died when my elder brother was 6 months old and my mother was married in 70’s to my father to look after my brother. We basically from agricultural family and we have 7 acres agriculture land that was our grand fathers property. My elder brother was not educated and failed in 10th and from then he is taking care our lands. My brother and me studied in rural college and with our own efforts we are now in decent job with decent salary in IT industry though not settled well. My elder brother married and has two kids. I am married and have kid. My younger brother not married. Now the issues is my elder brother wife threatening us saying they have full rights to claim all land as share than me and my brother as we are my fathers second wife son’s. She says according to law in 70’s only first wife has fully rights to enjoy the all 7 acres land. My father, younger brother and me want to divide the share equally between us. My younger brother and me are ready to give little extra share to my elder brother, as he is not educated but she refused. We are shocked by hearing her statement as we thought property share what we get my father will help us for our future. Could some one help me to understand the fact according to law. Is that true what my sister-in-law says. Do my brother and me has rights to enjoy our share. Please help me. Thanks in Advance Bala