My mom has advanced dementia and my younger brother has been exploiting her for financial gain! What can we do?

We need more information, especially the how, where and what issues. Who's caring for Mom? Your profile states you're caring for her and she's living at home. Is brother living with her? Are you all in the same state, county?

Has she executed powers of attorney? How is your brother exploiting her? Using her funds, manipulating her to give him money? How long has this been going on? What documentation do you have?