He was charged with murder Thursday.
All four members of Keith Caneiro's family were "victims of homicidal violence", the prosecutor said, adding there was no indication of suicide.
Authorities say Paul Caniero then set fire to his brother's home to try and cover up his crimes.
Paul Caneiro, the brother of a man found slain alongside his wife and children in a torched home last week, has been charged with murder, according to court records obtained by ABC News.
Obituaries describe the children as 11-year-old Jesse, who liked World War I and World War II history, video games, and sports, and 8-year-old Sophia, who loved ice skating, ninja classes, and making cookies with her nanny.
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Jennifer and Keith Caneiro were found dead at their Colts Neck mansion.
Prior to the alleged homicides, Caneiro had previously been charged with arson for setting fire to his own home in Ocean Township with his wife and two daughters inside.
Caneiro remains in jail on a charge of aggravated arson and was served with the new charges on Thursday morning.
That same day, about seven hours later, his brother's mansion in nearby Colts Neck, New Jersey, also burst into flames.
Paul Caneiro, 51, of Ocean had been in custody already on suspicion of setting fire to his own home November 20, the same day Keith Caneiro, 50, was found dead with his wife Jennifer, 45, their 11-year-old son Jesse and their 8-year-old daughter Sophia.
The woman's and the children's bodies were found severely burned inside the home, and authorities ruled out "a suicide angle", Gramiccioni said.
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An investigation into those businesses - and the financial dealings of the Caneiros - has been launched, the prosecutor said. He said the fire was started in two exterior locations at each end of the house.
Honecker previously maintained his client was innocent and said his client was "devastated by the news of his brother and his family's deaths".
Caneiro's wife and two daughters were home when he is accused of using gasoline to ignite their Ocean Township home, which is about 10 miles from his brother's home in Colts Neck.
Paul and Keith Canario were partners in a technology consulting firm and a pest control business in Asbury Park. Workers in the area frequently saw Paul Caneiro there.
The brothers were best friends who talked nearly daily, Honecker told CNN, adding he had no reason to believe there was animosity between them.
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