How can I check if an agreement is valid?

My mother is co-owner in agricultural property land along with her other siblings. Her elder brother (My elder Mama) is expired and his wife's, 2 son's and 1 daughter's names are now added by striking our his name. Now the news is one of the company located near these properties have taken land on rent for leveling (dumping sand/stones etc.). We don't know how Mama's family made agreement with the company for 10 years without consulting other co-owners (at least my mother and her younger sister don't have any idea). How we can get the agreement copy to see if fraudulently signatures or thumbs are given by unauthorized persons? Which steps should be taken to bring stay or cancel the agreement? And what precautionary measures we can take to prevent further misuse as the names can be removed through bribes or in fraudulent way?