How can I stop my siblings from borrowing from mom's savings?

Very lucky that you have POA. You don't have any choice than to control access to the funds. If your Mom is doing the lending, maybe you can convince her to talk with you about it and hopefully she'll understand you're looking out for her interests. If she doesn't get it, then you'll have to hang tough even with her, for her sake.

As an old retred lawyer, I can tell you that the answer is really simple. Your POA gives you the power to close you rmother's current account and reopen it under your name. Different states have different rules and/or laws. My suggestion is that you open the new account as,"In trust for (mom's name" the purpose being to make it clear that you are not claiming the money as your own but for a specific need. And BTW, when your mother comes to the end of her life remember to reduce the inheritance to the ones who took advantage of mom.

how do they get a hold of it??? do they talk her out of the money or what? You never know what is going to happen to our older loved ones... She should have what she has earned and saved.