Was he in custody at the time? That is an important factor. If they just walked up to him and started chatting it up and started questioning him but he was not "under arrest", then they do not have to give the warnings. Once he says something incriminating and they decide to arrest him, then they can give the warnings as to further questions. Also, they do not have to give the warnings if they are not asking questions. So if they arrest someone and they start blabbing about everything they did illegal but a cop doesn't ask a single question, there is no violation.
Your brother should consult with his own attorney who can carefully review when and what your brother was told (or wasn't told) to determine if he has a viable motion to suppress his statements. Challenges to the admission of statements are very fact-specific.
Your rights are violated if, prior to being questioned while you are police custody, you are not advised that you have the right to an attorney and that you may remain silent, and anything you do say may be held against you. Think carefully about that. If you say something incriminating but you are not in custody, there is no Miranda violation. If you are in custody and blurt something out without a question being asked, there is no Miranda violation. There are all sorts of variables here. Thousands of cases in courts across the country have interpreted Miranda. It is the subject of countless law review articles, and nearly endless debate. Many cases refine or limit what Miranda means. Your borther's lawyer will hopefully be aware of this, will parse through the facts, and do what he can to assure that your borther's rights are protected.
The response I have provided is general in nature, and does not create an attorney-client relationship. My practice is based in Rhode Island, and the law and practice in other states or jurisdictions may be different.