AirNZ should class your brother as "immediate family"

"Additionally, due to the high demand for seats in our HVC zone, from the 22nd of November 2012 the current seating over-ride functionality will be disabled. This means non Gold Elite/Gold/Koru members travelling with a Gold Elite/Gold/Koru member on a separate PNR will no longer be able to be seated in the HVC zone. These passengers have the option of being seated together outside the HVC zone or using Paid Seat Select to purchase seats together

We strongly recommend that non Gold Elite/Gold/Koru members wanting to access premium seating when travelling with a Gold Elite/Gold/Koru colleague book on the same PNR.

The only exception to the above is where a Gold/Gold Elite member is travelling and has an immediate family member that was unable to be booked in the same PNR, in which case please contact our Agency Support team to request seating in the HVC zone".

From: http://www.airnzagent.co.nz/wing-tips-nov12