Education and Training Act 2020, Schedule 20 clause 2(3)
Siblings (brothers and sisters) include:
half-siblings
step-siblings by marriage or civil union
step-siblings by marriage or civil union at the time one parent dies
children whose parents are in a de facto relationship
children who live in the same house and are treated by the adults of the house as if they are siblings.
In some cases, the Ministry of Education can also tell the school that other children are to be recognised as siblings.