Govt assigned Land(SC) issue with neighbors purchased from my brother

My Father and his brother are having a Government land which was assigned under SC quota in the 1970s and we have small houses constructed with the help of Bank loans at that time. Both the houses are separated with a common wall. After my Father's brother expired, his Son(cousin brother) sold their land with the house in their land to some others without intimating us. We both are not having the Government assigned land documents with us as they are in Bank due to loan due. The purchased person took a duly signs of my cousin brother's family members and some witness signs on the 50/- bond paper as a legal authorized person on the land and house. 1. Is it legal buying Government assigned lands ? 2. The purchased person haven't informed us before purchasing the land as we have a common wall which is a separation of two houses. Please suggest us.