It is very difficult, if not impossible, to appeal a plea because you give up most of your appellate rights during the plea process. Depending on when your brother pled, the better course of action might be to have an attorney file a Motion to Withdraw his Plea. However, there is a time limit on that motion so the attorney would need to know when the plea was entered so the Motion could be timely filed. Generally speaking, your brother would have to show that his plea was not voluntarily entered and/or that he was coerced to plea... which is a very difficult showing to make.
I agree with the answer posted by attorney Aaron Slavin. I would only add the following; as a general rule,unless your brother reserved the right to appeal a specific issue, once the judgment is entered, issues relating to guilt or innocence are not subject to appellate review; however issues relating to the judgment and sentence (for example did the length of the sentence exceed the statutory maximum) are subject to appellate review and may be challenged on direct appeal. Without knowing your brother's specific concern it is hard to determine whether the best course of action is a direct appeal or some other relief as suggested by attorney Slavin (al though I suspect that what attorney Slavin is suggesting is likely to be the most realistic and practical approach for your brother to get any relief).