He also faced a charge of assaulting a prison officer months before he stabbed the two family members.
He appeared over a video link at the Hutt Valley District Court this afternoon.
Eru and his family were at home on the morning of March 26, 2021, when he came up behind his stepdad, stabbing him several times in the back and forearm.
Rushing out of the house after the attack, and running onto the street, his brother and another family member chased after him.
Catching up to Eru at an intersection in Lower Hutt, he lashed out again, this time stabbing his brother once in his right bicep.
Fleeing the scene, Eru was later found to be at a nearby family member's house and arrested by police.
In a victim impact statement read aloud to the court today by Judge Tim Black, Eru's stepfather said mental health was the underlying issue for his offending.
He said that locking someone up isn't always the answer, but locking Eru up may keep his stepson and others away from danger.
He said he was glad it was him as Eru's victim, describing himself as a large man, and not another more vulnerable family member.
Eru's charge of assaulting a prison guard is from when he was in custody in 2020.
While in custody at Rimutaka Prison in October 2020, a Corrections officer was conducting cell cleaning and asked Eru if he wanted his cell cleaned.
Eru agreed, but when the door was opened, he rushed at the prison guard, punching him in the face with a closed fist with such force that his victim's tongue was cut by the impact.
Judge Black said Eru displayed the "paradigm of the face of our Māori prison population" and that he had suffered abuse, disconnection of cultural ties and deprivation in his life.
Judge Black said there was a consensus that Eru suffered from a long-standing mental illness, but what was less clear was the "nexus" or connection between that and his violent offending.