In a victim impact statement, Morehu said she forgave Reihana for what happened but defence lawyer Panama Le'Au'Anae said she would "not forgive herself for not keeping her children safe".
Before passing sentence, Justice Murray Gilbert said Parliament had recently increased the penalty for reckless driving causing death.
Reihana's learner's licence had been suspended after a previous drink-driving conviction, he had failed to ensure his passengers were safe - including his two-year-old daughter who sat under a lap belt with Morehu's eldest daughter in the front seat - and he disregarded pleas for him to slow down.
He also drove after drinking six to eight cans of beer and six shots of vodka and returned a reading of 157 micrograms per litre of breath - nearly twice the legal limit - when breath-tested by police at the scene.
Justice Gilbert said the manner of his driving was "nothing short of atrocious".
"Tragedy was inevitable and it was fortunate no one else was killed," he said.
Justice Gilbert said it was inevitable that Reihana would have been convicted and he only entered a guilty plea when it was clear it was the only outcome.
"It was an extremely belated guilty plea and belated acceptance of responsibility for what you have done," said Justice Gilbert.
"I must denounce your conduct and deter others from similar offending."
Reihana, 21, had earlier told his whanau in a restorative justice meeting that his mistake would stay with him forever.
"It's me who has to answer to God and accept responsibility. I swear I wished this had never happened. All I can say is I'm sorry, I'm sorry, I'm sorry that I chose to drink then drive."
Reihana's uncle, James Papali'i, who addressed the court on behalf of the family yesterday, said it was a particularly poignant day for Reihana and his family, who were to unveil a headstone at Ahipara for Reihana's son Tauwhetu who would have turned 3 earlier this week but died of cot death aged 12 days.
Reihana's lawyer Panama Le'au'anae said his client's fateful decision to drive dangerously and at high speed in a convoy had turned his whanau's Christmas celebrations into "a complete and absolute tragedy".
He said Reihana had accepted he would serve a lengthy prison sentence.
Reihana will serve at least three years and six months in prison before he can be considered for parole.