Is there suggested wording to put in a will to make sure my stepmother can't change her will after my dad dies?

My father's will leaves everything to his new wife, and her will divides up the remaining money between her kids and my siblings and me when she dies. The problem is she's free to change her will after he dies. Is there a way to solve this problem? I think the wife is entitled to the money when my dad dies, but if she changes her will before she dies, she can leave all the remaining money to her own children. I politely pointed out to my dad that she could change her will after he dies and he said, "But I don't think she will." Any suggestions?