Can a witness to settlement suit claim a share in same property

Sir my grandfather has 3female children including my mother who is elder. My grand father purchased land of 2cents from his savings and he settled in the name of his third child for which my mother and her younger sister signed as a witness, now my grand father expired and the third child is refusing to give money or partial land to my mother and her sister How can I take a legal action against them Land settled in August 2002. My lawyer is saying that the case will not stand in court since my mother signed as a witness in settlement deed.