my brother cheated my mother with property

My father gave a plot to my relative few years back for making a house only not for sell,and relative can sell plot to my family only,father gave plot at very low cost without doing any paper work. Now father is no more. The entire property was on my mother name. Now what happened my relative tried to sell that plot to outsider which was given to him only for house making porpose. So my brother transferred that plot on his own name by saying that he will give it back to my relative only when they will construct home on that plot or will sell to my family .that was fine,My mother agreed because this was the prime condition when my father gave plot to my relative. Now my relative is selling that plot in my family only means he is selling plot to me not to any outsider but my brother is refusing to give back that plot. What my mother can do now.