I've had a similar experience. If you can see her in an illegal situation (wandering the streets drunk is usually against the law), report it. I know it sounds harsh, but that might be the only way to get her in a safe situation. And it's likely the only thing in your power to do.
You should definitely consult an attorney who specializes in adult guardianship cases. It's very difficult to get, but you might luck out, and the law defines the requirements different than a family member would. My father-in-law has Alzheimer's and my mother-in-law wants to put him in a home, but she can't because he doesn't fit the description of incapacitated defined by the law.

I know it has to be painful and frustrating and I know you've brought your pain here to try to find some guidance. I wish there were a better solution. But you're still where you were at the start - all you can do is try to reason with her, but you can't save her; she has to save herself. Be strong.