Illegal property transfer between sons and daughters

My maternal grandfather owns 3 acres, and my late grandmother had 3 acres and other properties. Two years ago, he wrote a will dividing everything equally among his three sons and two daughters. Recently, under pressure from his sons, he created a new will giving them the majority and a smaller share to the daughters. The original(first) will is still with us. Is it legal to create a second will without canceling the first? Also, can my grandmother's share be transferred solely to the sons?