Brother takes advantage of 93 year old mother. What can I do?

The withdraws from her account every month will count in the lookback period for Medicaid. That has to stop.
Please explain to your mom that because your brother and his wife are both on SSI, they can get by just fine. They do not have to pay for health insurance because they're covered on Medicaid, their low income makes them eligible for things like food stamps, rent assistance, and utility assistance.
In all likelihood, they have more disposable income between the two of them than she does. Explain this to her in these terms.
Also, suggest to her that if she wants to continue with the financial assistance to your brother, that he and his wife should also be giving her proof of a clean drug test. Just to make sure.
Explain to her about the Medicaid lookback period. She might not understand about it.

Time for you to decide how much (and when) you’re willing to bite off. If you are willing to take over mom’s finances to cut off the leaching from brother, you have authority to do so, but it likely won’t be pretty. Mom won’t have access to money anymore, you’ll be paying her bills and brother will sink or swim on his own. I did this with my dad, actually while he was in the hospital and had no idea I was doing it. I changed all his bills to online being emailed to me, changed his banking to online with statements coming to me, forwarded his mail to my home, and cut off all the many charities he was supporting. When he came home I told him what I’d done, and was prepared with a binder laying out exactly his balances and bills paid. To my surprise he wasn’t angry and seemed relieved. With your mom’s growing issues, this need is coming sooner than later, it’s a matter of when you want to bite it off. Mom’s not going to be able to live in the current setup much longer, she’ll either need increased in home help or moving to where there’s around the clock help, so you’ll need to plan now for that time. Don’t argue with her about brother and money, that’s a useless loop that goes nowhere, just decide when you’re ready to act.

Wow what a great brother and son. And she's not going to listen to you. So, your options are limited. As DPOA, can you limit her access to money? She won't like it but that's going to have to be OK. She is likely to need her money for her own care soon enough. I'm surprised with dementia and at her age that she's still on her own. But some people can pull it off. I guess the only thing you can do when/if the time comes is to tell medicaid that all that money went to your brother. I don't know if they go after him for it or else she won't be able to get on it and then what? If he blows it all on drugs, he won't be able to pay it back so I guess all you can hope for is that you can stop it from happening and that she doesn't outlive her money.

I would let it go.

However, I would probably direct mom to home chef or some other prepared meals service. If she has money to give away she can buy her own food.

If she needs facility care in the future and runs out of money, well, choices have consequences and you are not obligated to be her solution because her choices have left her with terrible consequences.

My mom always favored my brothers, would give them anything and everything, yet when she needed help I was the only one around. Oh well, I don't have to account for others, only myself and that goes for every person on the planet.

You do what you are willing to do and don't do what makes you feel bitter or resentful. They don't care, you can't care more about her then she does, it never works out well.

How is she giving him money if you're handling everything with a DPOA? Is it not active? If it is, then take away the checkbooks and her source of funds. Order her groceries for her. Personally, I'd move her closer to you now to keep him a distance from her.

You know he's spending money on drugs, so reasoning with him about how he'd be ruining her chances to get Medicaid is pointless. You just have to take over all her finances, get guardianship if need be, and protect her.