You are in a horribly touchy situation (as you know). Obviously, she isn't able spot a scam. This happens to a lot of vulnerable elders, even if dementia is not present. She resents your intrusion (normal), so she gets angry.

I'd suggest that you get her to a doctor for some reason (like blood pressure or anything she will accept), and then alert the doctor that she is having this problem. The doctor may not find dementia, but he or she may be able to gently suggest that it's normal at her age to need some help with finances. If she doesn't want you to do it, since family can sometimes make them angrier than an outsider, he could suggest an accountant or some bank person to hand her funds. Third parties come in very handy for these issues, including driving cessation, as elders often look at their adult children as interfering.

I hope you can get help here. A pastor or rabbi could maybe convince her to get outside assistance.

Good luck and please keep in touch,

Carol