Is this legal for my step mom to do this to my father?

My father is on the innitial stage of dementia and would like to change his will but his much younger wife( my step mom) threatens to divorce him if he does not leave everything to her and make her the primary POA. My dad wants to divide his substancial estate between his 2 sons and his wife but he is afraid that his wife will leave him when he needs her the most. I would love to take care of my dad if she leaves him but I'm also curious how much she is going to get if she divorces my dad. They have been married less than 20 years and live in a state that divides ets equitably. He is old she is young, he made all the money and puts it in the marital ets she made her money and spens/saves it for herself. Is is legal for my step mom to prevent my dad from changing his will?