The boy has not been identified by the authorities because of his age, although he may be tried as an adult.
The teen was to have made an initial court appearance on Tuesday, but it was postponed until Wednesday as a judge weighed a request by a media organization to open the family court proceeding, which normally is closed, according to the Associated Press.
In the meantime, friends and relatives are struggling to reconcile the massacre with their conception of the Kologi family - large, loving and "picturesque," as described by the Asbury Park Press.
They had seen their troubles. Nearly a decade ago, Steven and Linda Kologi had filed for bankruptcy, the Press reported. But by 2017, they were still living in the same Long Branch, New Jersey, duplex and even prospering in some ways.
The couple, in their early 40s, had four children, according to the Press.
NJ.com wrote that 18-year-old Brittany - killed along with her parents - had recently finished her first semester at Stockton University, about an hour down the coast. "She was on the process to being great with whatever she chose," her brother Steven Kologi would write after her death, according to NJ.com.
As for the elder Steven Kologi - "my God, he'd do anything for his kids," a friend named Ronnie Pacheco told the Press. Steven Kologi had once worked as a postal worker and later took on a multitude of jobs and shifts to bring in additional income. "Some midnight to 9 in the morning," Pacheco said. "He did everything and anything to support his family."
A GoFundMe page set up by a family friend had raised more than $25,000 to help pay for the funeral expenses.
Friends knew Linda Kologi as a doting mother, New York's CBS affiliate reported. The whole family would often come into the Welsh Farms convenience store across the street, NJ.com wrote. Linda always looked happy and often bought a lottery ticket, the report quoted the shop owner as saying.
In warm weather on Sundays, the Press reported, the Kologis would sometimes show up at a local softball field.
The 16-year-old, some said, had gone through his own troubles. Kids used to tease him in elementary school, family friend Veronica Mass told NJ.com. He had a learning disability, and his grades at the time were poor.
His parents started home-schooling him a few years ago, Mass said, and the boy seemed to improve. "He learned to read, did his math," Mass said. He became a friendly, funny teenager. "He would tell jokes."
On the night of Dec. 31, Brittany was visiting from college, and the Kologis were gathered at home. The boy's grandfather was there, as was a friend, 70-year-old Mary Shultz.
Also in the duplex, according to the Monmouth County prosecutor, was a Century Arms semiautomatic rifle belonging to a family member.
James Roskowinski happened to be across the street about 11:45 p.m. He told the Press that he might have mistaken the pops he heard for fireworks - except that he also saw lights flash in the living room window and knew at once this was no celebration.
"Pop, pop, pop, pop," Roskowinski said.
Someone inside the duplex called 911. Before the year was over, police were lined up outside the Welsh Farms store, and lights and sirens filled the street.
The grandfather and the boy's brother were able to flee the duplex. The elder Steven, Linda and Brittany Kologi and Mary Shultz all lay dead inside the home.
Police said nothing of the boy's arrest, except that it was uneventful.
"We are investigating this and are confident that it's a domestic incident," prosecutor Christopher Gramiccioni said at a news conference. "It's completely isolated."
"Why would he kill his own mother?" a family member wondered to CBS in New York. "She was so good to that boy."
In the early hours of 2018, NJ.com reported, the younger Steven Kologi posted a sort of eulogy to his family on Instagram. He did not mention his arrested brother, but wrote of his parents: "Never once was I without a hot meal or a roof over my head. They made sure Christmas came every year even though they struggled financially."
"Please, please," he wrote, "remember to give the ones you love an extra kiss or I love you."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)