My stepdaughter has decided to take over care of my husband who has dementia as I am 82 and unable to provide caregiver status any longer. She has now taken over his SS payment and other responsibilities. I think she may try to take POA from me as well.
She cannot just take over as POA and it sounds like he can no longer assign her to be
Otherwise, guardianship by a family member or assigned by the courts is the only thing that supercedes PoA.
Most elderly spouses would welcome the help, as being PoA can become a very busy responsibility. Did your spouse reassign her as his PoA? If not, and you wish to take care of all his affairs (medical and financial) now and into the future, then you need to show your document to her and tell her to stop as she has no legal ability to manage his affairs and make decisions on his behalf.
Blended marriages are full of minefields such as this. If you don't clarify exactly what your authority is and that you will do all of it, then it will snow ball into a power struggle that will harm your husband's care. But you need to make a decision about your willingness/ability to do this much oversight.
I suggest you maintain a good and positive relationship with his daughter so you can have open communication moving forward.
Good luck to you.