How do I invoke POA over Mother when my brother is bleeding her dry?

Since he's using drugs, he's committing a criminal act. Call the police, anonymously if you have to. But consider how you could protect your mother from his druggie friends or suppliers.

As wWindy says, there's no invoking a POA. If you already have one, unless she is incompetent, you can't just take over her life. Apparently your mom has allowed and enabled this behavior for some time. You're probably not going to change things now.

I think I might call Adult Protective Services, tell them your brother is doing drugs in the house and taking horrible advantage of your mom. Hopefully, they will open a case file and make a visit or two to mom's home to evaluate her situation.

If they don't see reason to act, all you can do is keep your eyes open. If mom has been diagnosed with dementia, you might be successful getting guardianship over her. But, believe me, that would just open up another can of worms.

As for her money? You're most likely not going to be able to protect it without guardianship.

You can't invoke it over her, she has to be mentally competent and agree to a POA. If she is not, you're looking at guardianship. A court would assign you or someone else power to handle her affairs.