My Stepmom threatens to get attorney to stop me from making health decisions. Does she have any standing?

I am POA for healthcare and my brother is POA for financials. My stepmom doesn't agree with the treatment plan for alzheimers. She threatens to get an attorney and fight me. Does she have ANY standing as "the wife", as she says? I try to include her in decision making, but they are both 87 and she refuses to believe he is losing it. He hit me last week over not turning the car off fast enough. I am responsible for his safety and that of myself and her. I can't allow him to rant and rave all day, display the OCD behaviors and wear us both out! It is exhausting! What is my standing and what is her's? They have a prenup, but it doesn't say one word about healthcare.