How can a sibling change a legal living will to benefit themselves?

legally she cant because it wasnt her will to change her mother might be dead yes but it was still her mothers will and still her mothers final testament she cant change it unless she made her own will that would be that activated when she too passed but legally no she cant change another persons will ever if they passed if that was the case everyone would be changing peoples wills to match their needs using it for their own benefit

In summary the will is not hers to change she gets what shes entitled too in that specific will