How should I proceed with the immigration petition for my mother and my 17-year-old brother?

As the NVC has informed you, the petition that you filed does not include derivative status for family members. That is because you are filing in the immediate relative category for spouses, parents, and unmarried children under 21 of US citizens, and the law does not provide for derivative applicants. You can submit a new petition for your 17-year-old brother under the F-4 category for siblings of US citizens. His processing time will be approximately 13-15 years under current projections. Alternatively, your parents can immigrate and petition for him under the F-2A category for unmarried children under the age of 21 of green card holders. This category is currently projected to take around 2-4 years to complete. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.