Your brother needs a criminal attorney who is skilled in handling cases involving mentally ill defendants. The public defender's office certainly is able to handle these cases. Your brother can not be tried or sentenced if he is not competent, which requires an examination by a mental health professional. There is a special court called "Rule 11 Court" that handles these examinations. If your brother is represented by a public defender, make sure you ask if your brother is a candidate for a Rule 11 examination. Good luck.
NOTE: This information is offered for general purposes only and does not constitute legal advice about a specific legal problem. Neither the person asking the question nor any other readers should rely on the information as legal advice about their problem(s). This information is not attorney-client privileged and receipt of it does not create an attorney-client relationship.
Attorney Gieszl is giving you good advice, but you have to remember, when an individual is arrested, he or she is no longer working with the mental health system, he or she is dealing with the criminal justice system. For better or worse, the criminal justice system will tend to look at mental illness matters with a very calloused eye.
I would suggest getting an attorney who is comfortable working with the justice system, with whom you feel comfortable.
I am licensed in New Mexico and Pennsylvania, and therefore any discussion of issues related to other states must considered within that context. In addition, my comments are not intended to create a legal representation but merely to respond to the limited facts presented by the question. Any opinion herein is not meant as a precise statement of legal rights or as a recommendation of any particular course of action. A more complete legal review can be obtained through local counsel.