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She says she told Pacific Christian School in Mangere her little brother was being bullied, but was ignored.
The 23-year-old says if the school had listened to her concerns, the stabbing could have been avoided.
In an email to the Ministry of Education, the woman alleges the school refused to listen to her and her lesbian partner when they complained about the bullying.
She said she was told in a meeting with the school that her same-sex relationship was "not of God".
The email was sent late on Friday afternoon.
Yesterday, the ministry was unable to check its computer system to confirm receipt of the email, but Katrina Casey, its head of sector enablement and support, said any complaint would be investigated.
"If we do receive a complaint that involves either of the two children involved in the incident, we will pass this information to police," she said.
"If any matters relating to bullying complaints are outstanding once the police investigation is complete, we will work with the school to address these."
Pacific Christian School principal Lisita Paongo did not reply to requests for comment.
The complainant said her little brother had been "constantly bullied" at school in preceding months -- an allegation a source close to the family first made to the Herald last week.
"I don't agree with what [my brother] did to the poor student, but I know that if my concerns about my brother were taken into consideration, this whole incident could have been prevented.
"There was a tragic incident that happened at the school ... which I think could have been avoided ... " she wrote to the ministry.
The 11-year-old was taken from the school by police on Tuesday afternoon following the stabbing and has been in Child, Youth and Family care since, while police investigate.
It is understood he stabbed his 11-year-old classmate in the temple with scissors during a fight. The other boy remains in a critical condition in the Starship children's hospital.
A teacher was not in the classroom at the time of the fight.
A source close to the family told the Herald last week that the boy would sometimes come home in tears from the bullying at school, which had been going on "for over a year".
The sister's letter said: "The tragic incident that did happen at the school was because of bullying. At this time I am very disappointed in the school's principal."