Any number of subsequent dependants can be added. Your brother is the principle alien and all dependents qualify under him. No problem, just call it to the attention of the NVC when the time arises. You would be well advised to have an attorney assist you in the consular processing portion . . . it gets complex.
I agree with the other two responses. Inform the NVC of the new addition. Also, when his priority date becomes current, he will have to list all family members immigrating with him.
Yes NVC will require the birth certificate and then they will generate an additional fee bill and forms request at the Department of State stage only. No need to do anything until the I-130 is approved and routed to National Visa Center.
Lynne R. Feldman, Attorney at Law
Concentrating in Immigration and Nationality Law
2221 Camino Del Rio South, Suite 201
San Diego, CA 92108
phone: (619) 299-9600, facsimile: (619) 923-3277
Formerly Adjunct Professor -- Immigration law
University of Illinois College of Law