You have full legal authority over your mother's finances. The question is, how do you enforce the POA in the real world, against a daughter who seems intent on subverting
the POA? That is not an easy question to answer. In theory, you could take your daughter to court, and seek an order prohibiting her from interfering with your legal responsibility under the POA. But that would be, at best, a cumbersome and expensive process. And even you obtained an court order, it would not be self-enforcing. If your daughter violated the order, you'd have t return to court yet again.So, in my opinion, court proceeding are out.
So you need to find practical, real-world remedies. I do not know how your daughter manages to get your mother (her grandmother) to give away money that are not in her best interest. Is it sensible to prohibit your daughter from seeing your mother? Another possibility is to use your POA power to transfer your mother's money into accounts you alone control. Of course, you would have the legal responsibility to use these funds only in your mother's best interest. There may well be other practical steps you can take to protect your mother's interest. Unfortunately, those steps are not ones a lawyer can help you with.