Can a inherited property release deed be cancelled?

Dear respected Lawyers: My mother has signed to a registered copy of inherited property release deed to her brothers, in that there is no mention of property of her father's mentioned in that release deed and it has been signed by her with high-pressure and with stress from them after speaking to them in mutual that they will give one acre of land out of 50 acres of land and this was not mentioned in the release deed, but after two years of release deed. They are not talking about giving the one acre land to my mother as it is not mentioned in the registered release deed and they cheating her. Please tell us what to do and we have approached the court and what rules to be followed. Please help us as my mother is fully depressed about her brothers' way of gesture towards her. They are not talking to her and not inviting to her to any functions, it is okay that is not bothered about the function but they cheated her by not giving the one acre of land out of 50 acres of land as they have talked earlier before registering the release deed.