We've been there OP. Similar situation with BIL using deceased FILs card to withdraw money, pay for (his) bills etc, etc. MIL had given him permission to withdraw money on her behalf, but nothing else. She never looked at her bank statements as she was 'old school' and all the finances/bills were the responsibility of the man of the house (in this case BIL). He gradually took advantage of the situation.
He had upwards of £20k over the past 8 years or so - even though it was with (dead) FILs card, so fraud. Came to a halt when MIL went into residential care and DH applied for Deputyship (no POA in place). Once we took over the accounts it became obvious what was happening - no-one had removed FIL from the joint account. BIL was supposedly executor of the will but none of the normal things were done - not even advising the solicitor of FILs death for probate reasons.
Not sure if you've said, but if one of your siblings has access to your mum's account, has a third party mandate been set up? If so, can you reverse this? Does your mum have capacity to make this decision?
In our case I wanted to go to the police as, yes, it was abuse, theft, and fraud. For many different reasons this didn't happen and I wish I had said something. Got more of a backbone now and wouldn't hesitate.
You need to do what you think is right OP - no matter how difficult it is. I hope you manage to sort things out. It's shit when people take advantage like this, and even more shit when you feel powerless to stop it. Good luck.