Paul Caneiro methodically “hunted” down his sister-in-law, niece and nephew after executing his brother outside his New Jersey mansion — so the dad couldn’t protect his family, a prosecutor told jurors during opening statements at his murder trial Monday.

Paul Caneiro, 59, is on trial for killing his business partner brother, Keith Caneiro, 50, his wife Jennifer, 45, and their daughter and son, Sophia, 8, and Jesse, 11, on November 20, 2018, and then setting their luxury Colts Neck home on fire to cover up what he’d done.

The Caneiro brothers ran an NJ-based tech company, Square One.

Paul Caneiro “snuck up on the family while they slept,” in the middle of the night, prosecutor Nicole Wallace told a jury in Monmouth County Superior Court in Freehold, NJ.

He “waited in the shadows” for his younger sibling to come outside and check the generator after he cut the power and turned off the backup power, Wallace alleged.

Wielding a handgun, Caneiro “pulled the trigger six times” and executed his brother “on the spot” so he was “out of the way” and couldn’t protect his family, the prosecutor said.

Caneiro then set his sights on his sister-in-law, niece and nephews — and “hunted them down,” Wallace claimed.

He stabbed and shot his Jennifer and then repeatedly stabbed the kids — stabbing Sophia “17 times over her entire 45-pound body,” the prosecutor alleged.

Caneiro then started a slow-burning fire in the basement, and the two children, still breathing, inhaled smoke as they bled out, Wallace claimed.

After that, he went home and set his own Ocean Township home ablaze to make it look like the whole family had been targeted, Wallace claimed.

However, he ensured his own wife and kids managed to escape the fire without injuries.

The entire plan was motivated by “greed and desperation,” Wallace charged.

The accused kin killer — who wore glasses and a sweater with a button-up underneath and looked downcast as the openings got underway — carried out the slaughter because his younger brother found out that he was stealing tens of thousands of dollars to fund an affair with a mistress, prosecutors claimed.

Keith Caneiro called Paul the day before the murders, confronting him about why money was missing from a trust fund that was being used to pay Keith’s $3 million life insurance policy premiums, officials claimed.

Paul was the sole trustee of the fund.

Paul dodged the grilling, claiming he had a migraine. The next day, he killed Keith and his family at their $1.5 million manse before his swindling could be confirmed, prosecutors alleged.

Keith was allegedly shot in the head four times, while his wife was shot in the head. The children were found with stab wounds and burn wounds.

Paul has insisted on his innocence on 16 charges of murder, aggravated arson, weapons possession, theft, hindering his arrest and related crimes. He faces up to life in prison without the possibility of parole if convicted on the top charges.

He’s been jailed without bail since his arrest on Nov. 21, 2018.

The case has faced years of delays for myriad reasons, including the courts’ slowdown during the pandemic, Paul’s lawyers seeking to keep DNA and surveillance evidence out of trial, and a new lawyer joining his case last year.

A panel of 12 jurors and four alternates will be tasked with deliberating at the trial, which is slated to last up to 2 1/2 months.