Can you guess why this has happened? Is it a hangover from before your divorce – and if so does your mother still have the capacity to make a new POA? Does your mother approve more of her than of you, and this will still be her choice? Is your ex-wife likely to be willing and able to do the work required for her ex-MIL? Does your mother think this will force you and your ex-wife to co-operate with each other and so get back together – and if so is this a realistic wish? Have you read the POA document – in particular is there an option to replace your ex if she does not wish to act? You need to talk to your mother, try to find out why this is happening, and help her to change it if you think if will not work out well for her. Clearly she needs to change it, unless she is no longer competent or unless your mother approves of your ex more than of you. If it gets to the point where you are considering applying for guardianship, you will need to know the answers to all these questions, so start with sorting the situation out first.
Somebody else's having POA for your mother doesn't stop you doing anything you want, within reason and limits. All it means is that your ex wife will be making key decisions on your mother's behalf, which she must make in your mother's best interests. By "care for" your mother, what exactly do you mean? What is it that you want to do that you think you won't be able to do?
Guardianship if you feel wife is not a good choice. You can use Moms money for the cost. Your ex can always revolk her POA but u would still need guardianship if Moms Dementia is too far along.
If your mother mentally incompetent that she cannot revoke the first POA and create a new one?
If push comes to shove, you can apply for guardianship, if your mother is not competent.
It's a negotiation, that's all.
Have negotiations not been going well..?