Levin man Eric McIsaac has been sentenced to life imprisonment after he admitted murdering his brother, 10-year-old Alex Fisher, at Waitarere Beach in October last year.
McIsaac must serve 14 years without parole for the murder.
McIsaac, 26, pleaded guilty to the charge in May.
The court was told how McIsaac and Alex were spending the night at Waitarere Beach, near Levin, last October.
After Alex fell asleep, McIsaac hit him twice in the head with a wood splitter, killing him immediately.
During sentencing the Crown said it was not clear why this happened because McIsaac would not say, but it emerged McIsaac believed he was dying from cancer and that his half brother had been abused.
McIsaac also admitted two other charges: possession of an offensive weapon (a knife) and burglary of a Waitarere Beach property between 8 October and 9 September.
Alex's body was found at Waitarere Beach after a three-day search in October.
More than 100 people took part in the search, helped by helicopters, police dog units, search and rescue teams, and Air Force personnel.
Sentencing McIsaac today, Justice David Collins said McIsaac's offending was impossible to understand considering his close relationship with his victim.
"He was just ten years old, he was asleep at the time you murdered him, he had no chance of fleeing, or defending himself."
The outcome, he said, was the worst ordeal any family could go through.
Justice Collins took three years off the normal non-parole time of 17 years because of McIsacc's early guilty plea and because of his mental condition, though there was no formally diagnosed mental illness.
Alex Fisher's family have paid tribute to him outside the High Court in Palmerston North.
Detective Sergeant Neil Forlong of Horowhenua police read a statement on behalf of the family.
"Alex was much more than the sad event surrounding the end of his life, we his parents, siblings and family had nearly 11 years of memories of a treasured and much-loved boy who will forever live in the hearts of so many.
"We wish to focus on these memories and do the one thing that matters most - remember Alex."