My siblings are draining my mom's money. Is there any other way to protect her?

First call Adult Protective Services and ask for their advice. Next, try to reason with your mother. Remind her that she intrusted the PoA and the financial responsibilites to you and if she wants to continue receiving great medical care, she needs to think of her future. Tell her what you told us about how much ALFs and NHs cost...my Mom still won't believe it! The reason she thinks that it is her responsibility to be the family "bank" is because you are doing all your hard work "pro bono." I know the feeling.
Next, inform her that no more "loans" will be going out to the sibs. They are adults and can handle their own lives. If she refuses, tell her that you can no longer be her caregiver if she will not give you the respect due one. You are just tying to protect her assets from future scams.
If she is suffering from dementia, I would take away the check book. Talk to her bank and see if there is a way to place a password (that only you know) on her account, put a maximum amount on any check written, and stop any wire transfers. You have the PoA...there shouldn't be a problem. I also use online banking so I can monitor Mom's accounts.
If your Mom has major liquid assets just sitting in a checking or savings account; these are the most vulnerable. Talk to an attorney about a trust account, with you as the trustee. Or place funds in a CD so that she is getting interest and no one can access them.
It is incredible that you have to do this in addition to your caregiving duties. I am in the same boat and am tired of having to run interference between my Mom and my sib. Long ago, I made her promise not to give my sib money....never worked. So you have to take drastic measures. If she needs financial assistance somewhere down the road...guess who will be have to take care of her? Do whatever it takes to protect your Mom and tell the sibs that the "bank is closed."

I think an important point to add to this discussion is that if a parent is within 5 years of having to go in a NH and will then need to apply for Medicaid, any "gifts" will be gone over with a finetooth comb during the Medicaid app and the parent will be penalized, leaving the children holding the bag.

This my was prime concern when my brother started "helping himself" to our mother's money. I was under an enormous amount of stress worrying that the NH would kick my mother out on her a** when Medicaid found out about all the "gifting" that was done. I was able to put a stop to it when I discovered it, and now the last of the gifting is almost five years behind us. My mother has enough money to pay for her NH for another 1.5 years, so we'll be safely past the five year lookback period when she applies. My blood runs cold to think what could've been if I didn't discover my brother's theft. I only found out he was doing it when I signed up online to check my mother's credit card bills and monthly checking account statements. I think I aged 20 years in one day when I made that horrible discovery! I didn't stop crying for a month, and then I started getting my mother's funds secure (I'm POA). Good luck to anyone in this situation.

I have relinquished POA, but will share the info about the Medicare lookback w/ the sibs. Maybe that will motivate them to lay off coercing Mom for $. BTW thanks for sharing that you cried for a month. I thought it was just me. and yes 48 is the new 68.