You need to speak with an attorney in the area of Wisconsin where your mother has moved your brother. Many county bars have some type of legal advice program that can initially screen your question and direct you to attorney volunteers (who sometimes accept a reduced fee).
This is a general answer only and you should seek the advice of counsel to address facts specific to your circumstances.
Attorney Hackard has given you good advice.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
Can you get him back to California? The Superior Court in California, has no jurisdiction over individuals living outside the state. Once he's returned to California you can petition for Guardianship. His nomination of you certainly carries weight with the Probate Judges.
No legal representation exists by virtue of this answer. It is recommended that you contact an attorney directly for a more complete response.