Can personal items given by a parent before the parent passes be taken from you after they have passed?

Several years before my mother passed she gave me two of her personal necklaces. One was a graduation present and one was a birthday present. In her will she stated that I was to receive all her necklaces. Now my father's health is failing and both of my older sisters are starting to fight over money and property. They both want the necklaces that she gave me. Is there anyway they can be taken from me after my father passes? I.e. One or both sisters try to get my dad to change my mother's wishes in her will, by stating in his will that the necklaces should now go to them and taken from me? ( I have possession of them)