My sister and I have a joint power of attorney for our mom who has dementia. My sister is stealing and APS won't do anything. Any suggestions?

There is an argument that POA gives the power to make gifts (including to oneself). A trust would establish guidelines for a fiduciary responsibility to protect your mom.
If it is a joint POA, are you required to act together? If not, you could take the money and put it in an account under your name to protect it from her. Or, you could set up a trust and become her trustee and POA under the trust.
Talk to an elder lawyer so you can understand the legal obligations of the POA and your options for action. Consultations are usually free for the first conversation.

Do you know what it is you were hoping they do? I'd probably call APS again. If they can't won't help, find out why, and what steps they suggest you take. Does your Mom have an attorney? Does your sister have a legal right to keep you from your mother's home? Also, remember that thievery and fraud are crimes, so you probably have more recourse than you imagine.

Can you afford a one-time consultation fee with a lawyer? A qualified, ethical and compassionate ElderLaw attorney can help immensely. I'll stand aside and see what others suggest. I just wanted you to know, while you wait, that there are many people out there who have been where you are and can help.

Part of the problem may be that you aren't in the area ("I am having problems seeing my mom"). Is there any way you could move mom closer to you or you move closer to her? Sis is free to work her deceit because you're not around.

I agree with Riverdale. Providing you are on mom's bank accounts, take most of the money out and put it in a new account (set up for only that purpose) in just your name.
I would suppose that your moms' SS check is deposited once a month, so see if you can beat your sister to it, then put it in your account also.

Because of her choices, you have to play dirty since your sister can't be trusted.
Alert the credit card companies to cancel the cards due to your mom's dementia. There must be a central credit reporting agency you could call to flag her name for future possible attempts.

Report the financial abuse to Adult Protective Services again, but this time demand to speak with a supervisor. Have the date and name of the intake person you spoke to ready.

Go to your nearest Senior Center and ask if they have an elder law attorney available for a reduced price (or free) for a consultation. They might have some ideas on how you can help your mom be protected from fraud.

So sad when this happens. But you need to protect your mom from your sister, the wolf.

Could you go with your mother to the bank and open a new account and then transfer the funds to it
It is just a thought since you are enduring such problems. I wonder if she is on probation at all. Not quite sure what you could do but if you could find anything out perhaps you could report it to probation. Ideally it would be best to have your sister removed from the account but that would require some legal involvement. So sorry you are enduring this. I know some of the feeling. It is much more painful when family is involved with so much negativity towards agonizing situations.

There is National Academy of Elder Law Attorneys, search their site for pro
bono attorneys. I believe they have a list by state. Anyway they would be a good
start. Don't delay!! Good luck! Sorry you have to be going through this :(
(((hugs)))