Michael Bever was 16 when he and his older brother, Robert, stabbed their mother, father, two younger brothers and five year old sister to death.
Two sisters survived the attack, which was described as the worst single criminal event in the history of Broken Arrow city in Oklahoma.
Now 19, Bever was convicted in May of five counts of first degree murder for his role in the killings.
The teenager’s lawyers argued that he had been led astray by his older brother, who pleaded guilty in 2016 and was sentenced to life without parole.
While prosecutors urged that the horrific nature of his crime merited a sentence without parole, Michael Bever was granted life with parole, eligible after 38 years.
Officers were alerted to the 2015 killing spree with an “open 911” call, in which the caller did not speak.
They were unable to get in through the front door of the house, but heard moaning and found the boys’ injured sister before dragging her outside over fears the killer might still be inside.
Commenting at the time, neighbour Patricia Statham said: “It certainly is shocking. I’m shocked. I feel so bad for everyone who walks into that house.
“You can see it in the faces of the officers when they come out.”
During Bever’s sentencing a lawyer read a letter from the adoptive mother of the surviving sister, and the 2-year-old who escaped unharmed, describing how the girls would spend their lives “looking over their shoulders”.
The older sister, who still bears scars from the attack, “is terrified of that thought that her brother could possibly be paroled and what he might do to her,” the letter said.