Ask if they were a prosecutor previously - these usually make excellent criminal defense attorneys because they know the ropes on the other side. Ask how many cases they've handled, how many cases like yours they've handled and what result, ask for references, ask other lawyers in the community if they know the reputation of the lawyer, look at news stories on the web to see if their name comes up in any high profile cases in your area or nationally, check with the state bar to see if they have a record of discipline, check here on avvo for client or peer references.
Disclaimer: It is impossible to evaluate a legal problem without a comprehensive legal consultation and review of all the facts and documents at issue. Consequently, this response does not constitute or establish an attorney-client relationship but is offered for general informational purposes only. Laws differ from state to state and each case turns on facts specific to the case and parties thereto, thus this answer does not constitute legal advice, and should not be relied upon as anything more than a starting point or suggestion that the questioner seek professional assistance from a practitioner in your state or county/parish, practicing in your area of law.
I agree with Ms. Wilson's analysis in large part, but one of the best ways to tell who the best appeallate lawyers are is to see who has won the most appeals of this nature. You may have to do some real research, but it will be worth it in the long run. If you don't know how to do that, then speak to a local appellate attorney and ask him/her to do a computer search of those similar cases that have been won by some attorney and then go and interview him/her for the job.
Remember, you didn't say he didn't do it. You said the sentence was too harsh. That's a big difference and should make a difference in which attorney you hire. Also remember you may want to find an attorney who is familiar with the Prison system and how to work it once he's there, since there are a lot of things that can be done in that venue.
You will want to make sure the attorney has substantial experience with appellate practice.
Look for an attorney who is a career criminal defense attorney, and is aware of the procedural issues, which might impact the due process that your brother received.
Good sources for potential referral? I would contact the local Federal Public Defenders office and ask if they might be able to refer you to some attorneys handling federal appeals. Typically these attorneys are also handling state appeals or know those who are well known in the field. (PS In my humble opinion the fact that an attorney is a former prosecutor does not say anything one way or another about their skill or dedication to their clients. . )
Whatever you do, you'd better schedule your interviews as early as possible because there are time limits during which time motions to reconsider or notices of appeal must be filed. If those papers aren't filed on time, an appeal is sure to get tossed out as untimely.