You seem to be confused. It is not your and your brother's income combined. If your income stand alone is not enough, then you must obtain a joint sponsor. That joint sponsor must by himself make above the 125% of poverty guidelines for the particular household size. It does not look that your brother qualifies.
This is not legal advice and a client attorney relationship is not created.
Read the detailed NVC packet instructions you must have received. The instructions for the Affidavit of Support are all in there. If you are still confused after that then call the NVC's public inquiries line. From the facts you cite your brother does not seem to meet the Affidavit's income requirements, you will need to find another co-sponsor who does meet the financial requirements.
Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.