My mom has dementia and my sister is taking and selling all of her things. What can I do?

It sounds like your sister has acquired guardianship, but you ae the primary caregiver.  Unless you are getting paid a fair amount, I would tell sis, unless you start sharing information with me, I am not giving any more unpaid labor

You say you saw APS already? Did they basically sign off on your Sister having guardianship? She has also Representative Payee from Social Security? My experience with that is that SS is VERY careful about conferring that, and that there is both letters from MDs, etc. Yearly accountings must be made. The person must be proven incompetent. Guardianship is also difficult to get. It is generally given by the court. What was the outcome of your Fraud accusations through SS? Also, of course your mother's car should now be sold, and assets go toward her care or into her accounts, as she can no longer drive. Of course Sister would be on ALL of Mom's accounts and financial records now as she is acting for your Mother as her guardian. I wonder also if sister is now downsizing so that Mother can enter care?
If you no longer wish to be closely involved do give sister the date that you will no longer do caregiving for Mom, so that she can arrange same.
I am assuming, with your mention of APS that they have signed off that they do not feel they should open a fraud case?
You might consider seeing a law attorney with the evidence you have of abuse. That is likely last stop on this as all other things have been done. You may want to move away from the situation if you find no satisfaction under the law.
Wishing you good luck.

Can you afford to consult a lawyer who deals specifically with elder care issues at least for a consultation? You might try several to try for a better rate. This reeks to me of elder care abuse. I believe depending on where you are there are sources to possibly help should you make this known. At the very least your mother is being denied medical treatment not to mention numerous other issues such as lack of clothing,likely needed medications etc. She could be left destitute with no place to reside. You could go to her bank and speak to an officer and state that in the interim of the appointment of your sister you have strong indications of fraud. Any legal advice you are able to contract could well be worth it. If she has not been seen recently by any medical providers despite Covid there is at least the fact that there has been no communication that is definitely needed for her conditions. Medical fraud is generally not taken likely. Are there any insurance companies involved? This may eventually sever relationships with your sister but from what you describe she sounds spineless which sadly is not an uncommon occurrence in many situations similar to yours.