Sometimes becoming a bilingual or Māori speaking household can cause negative attitudes from others. Surprisingly or not some of those closest to you might have the strongest opposition to your decision.
The best way to start changing this is by having a conversation with your extended family and friends about what you are doing and why. Most of the time people will feel that the child might be disadvantaged in some way or that they might be limited in how they are able to interact with you and your family. Everyone can play a part in fostering bilingualism. For 'English-speaking' only family members this might mean that they take charge of ensuring that the English of that child continues to develop and grow and vice versa for Māori speaking family members.