What to do when co-POA withdraws large amounts of money from parents' bank?

Mom has dementia and can’t remember what she said 5 minutes ago. Her adult (age 58) son just moved in bc he had nowhere to live. He’s been living there for almost a year, not working. Says he has a job, then didn’t take the job bc there was a better offer. The, he doesn’t take that bc there’s a better offer and so on. Co-POA just took funds to buy him a $10,000.00 used car bc he was using hers. Mom doesn’t understand that she can’t spend more than her income. She just says dad left me with plenty of money to do what I want. I told her that would be the case if she wasn’t giving her money away. She says she isn’t. I told her large checks are written and withdrawn on her account and she says, "I don’t know anything about that because my memory is bad but your sister takes care of everything." Then said that my sister wouldn’t steal from her. I said we need to find out what’s going on. Then mom says “ we’ll she took me to a lawyer, I’m not sure why but when I had to pay him, he said go to the bank across the street and get cash so I know I took cash then.

So, I confronted my sister who won’t answer me.

I also got an alert of the mortgage not being paid. I told mom and she said my sister pays the bills and is so overworked she probably forgot, but it is 3 months late. So I paid it. I contacted my sister but she will not pick up the phone.

Between my sister and brother they are using my mom’s debit card multiple times a day, wiping her account out.

My brother uses it to take his girlfriend out and let’s her stay over in my mom’s house, but my mom doesn’t know it.

I don’t know how to stop this. Please help if you’ve been in this situation. My other issue is they tell her not to tell me things. However my mom will say “I’m not supposed to tell you this but…" This is not healthy for dementia patients. It’s horrible.