My Mom's caregiver is accepting assets and monetary payment, but Mom's mental capacity questionable. What to do?

You may need to seek guardianship. Call Adult Protective services snd tell them caregiver is exerting undue influence. You will need to contact an eldercare attorney.

Caregiver may be telling mom that you all want to put her in a home and that is why she, mom, needs to side with caregiver. Involve mom's physician in this situation. S/he can advise you as to mom's capacity.

Thank you for your advice. To complicate matters, my mother is isolating us, her children, and not being upfront with what is happening with the caregiver. She has even asked the caregiver not to talk to us. We are trying to keep the channel of communication open between us and the caregiver, at the same time keeping the caregiver at arms length. Is there a legal resource?

Oh...your mom is paying taxes for what? The caregiver is private and not through agency?

See that your mom has a non-revocable trust established with herself as beneficiary. You need DPOA so that if she is/ becomes incompetent you can manage her affairs and can start slowly if she is reluctant. You need to be diligent in looking out for the people she surrounds herself with. In particular, this caretaker of you cannot get her removed you will need to be careful in your dealings with as she sounds like a manipulative person..,look up the definitions of how this personality type operates. She may not be above turning your mom against you and if you are not aware that is happening it could be too late. Get family involved to minimize isolation if that is occurring. Run credit check. Recording video and cameras may be against the law in your state, unless your mom gives you permission. Though may be helpful to hear what's being said when you are not around. Document, document, document. Use your cell phone to make it easier because it's a lot of wirk.
I'm sorry for your trouble. I'm dealing with all of this.
It's emotionally draining. Be sure to take care if your self and your family and watch out for the dysfunctional vortex that wants to pull you down. Approach the problem(s) logically and reach out to experts in your community. You can get advice from elder services without filing a report...just ask them for that.

Who is this caregiver? Who hired him or her? Are they being oaid, or was this coin collection in lieu of payment?

If this is an outside caregiver, you fire them. If this is a family member or "friend" this action is a clear indication that your mom needs a higher level of more professional care. Get her a complete physical and mental workup so you and family can determine her needs.