"he is getting money from her account" ..in what way?? legally?? if not she needs to file charges on him. If she won't there's not much you can do.. unless you or someone else has power of attorney
I'm wondering if there are other siblings or family members willing to do an intervention on your brother?
You stated she's of sound mind. So given that bit of information and short of an intervention. Like it or not there's not much you can do, and she's free to do as she pleases with her money, resources, etc.
However, an intervention could backfire and make her feel even more pity for him.
But it sound like he needs one regardless.
Maybe you could get a counselor or someone to talk to her about being manipulated by addicts?
However, if she's of sound mind there's nothing much to be done legally
that's her son. If she feels like giving him what he wants there's nothing to be done about it. So the other approaches maybe best.
She maybe of the mindset that she's nearing the end of her life and would rather help someone that needs it than to say no...
and it's her assets to do as she wishes with..