In Maryland, your brother would have to make a written request with the PD's office to file a Motion to Correct an Illegal Sentence, Motion to Modify, Writ of Actual Innocence, or a Petition for Post-Conviction Relief. Upon receipt of the request, a PD would review the case and if suitable, file one of the aforementioned Motions. The State would then file a response. After the State's response, the local court may schedule a hearing to hear arguments from both sides. It's important to remember that the Defendant, not family, would have to make the request to OPD.
Your brother could also hire a private attorney or file his own Motions "pro se," however, having observed dozens of self-represented appellants, I would strongly recommend against that option.
The Department of Corrections does internal audits of prisoner sentences - usually closer in time to the sentencing, but it can happen years later.
Your brother may have plead to second degree for 15 to life, but the paperwork reflected a first degree plea or a 25-life sentence. If it's a clerical type error, the case will be referred back to the sentencing court for resentencing to clear this up.
If there is something more to the situation that made the plea and sentence illegal, then it may give him grounds to withdraw his plea or have it vacated. Without knowing what specific error was, I can't speculate as to what other options he has.