Your grandmother, as the absolute owner of property left to her by your grandfather, has the right to bequeath it to whomever she chooses, including you and her daughter. Your stepmother and stepsister are unlikely to successfully challenge this new will, as your grandmother has clear title. While a will takes effect after death, making it potentially challengeable then, executing a gift or settlement deed now could offer a more immediate and secure way to transfer ownership while safeguarding her interests.