At the time of the attack on February 9, Cooper was staying with his mother and brother at their Flaxmere house and they had become concerned about his behaviour.
They suspected Cooper was using methamphetamine.
The 16-year-old also lost three teeth when he fell against a bed headboard after being hit by the hammer.
He was flown from Hastings to Wellington Hospital for an operation to repair his skull and relieve pressure from a small brain bleed.
Their mother suffered fractures to her cheek, nose and jaw and was also taken to hospital. She needed eight staples to join her split scalp. Her eyes, face, chest, arms and leg were swollen and bruised.
When police found Cooper, a Mongrel Mob member, the next day he was too impaired by drugs to be able to comment.
Cooper appeared for sentencing on two charges of wounding with intent to cause grievous bodily harm, as well as unrelated charges for discharging a firearm or air gun, two of unlawfully carrying a firearm or air gun, as well as attempted theft and intentional damage.
The firearm charges related to July last year, when Cooper went to the house of a former girlfriend. He was wearing a black leather vest with a Mongrel Mob patch.
He confronted his former partner's stepfather, whom he did not know, telling him "this is my f...... turf" and using a racial slur.
He said he was going to take the stepfather's Ford Falcon car but could not get into it as the doors were locked. He also tried the doors of a neighbour's car, saying "everything is mine, this is my territory" before walking away.
About 11pm, Cooper returned and threatened his former partner with a firearm or air gun outside the house.
When her stepfather came out, he yelled another racial slur and discharged the weapon.
A Crown summary of facts said a bullet narrowly missed the stepfather, went through a garage door and lodged in the wall of a room where a three-year-old girl was in bed.
Reviewing Cooper's background, Judge Collins said he was "really just a boy" when he was jailed for six years in prison for serious offending. He spent nine years in prison as a teenager and young adult.
He sentenced Cooper to six years and six months in jail for the charge of wounding his mother, six years for the attack on his brother, and lesser sentences for the firearms charges, to be served concurrently.
The judge said he had considered a minimum non-parole period but Cooper's counsel, Nicola Graham, had "persuaded" him to leave his release date up to the Parole Board.
Lara Marshall prosecuted for the Crown.
"There is already on your part a willingness to rehabilitate," Judge Collins told Cooper.
"You realise that you will have a life after prison and you don't want to continue to offend."
