The younger brother, 17-year-old Michael Bever, faces the same charges and had not guilty pleas entered for him on all counts.
Sister Crystal, then aged 13, was critically injured but survived the attack at the family home in Broken Arrow, about 15 miles southeast of Tulsa, police said.
Tulsa County District Attorney, Stephen Kunzweiler gave the rationale behind reaching an agreement with Robert Bever and not pursuing the death penalty.
Robert was seated first, at the defense table. "Those children deserve a life, and I am not going to saddle them with what I know the reality of a death penalty case to be".
Robert Bever and Michael Bever who were 18 and 16 at the time had bolted out the door as police arrived, but were tracked down by a police dog and taken into custody.
This photo provided by the Broken Arrow, Okla., Police Department shows Robert Bever, a suspect in the stabbing deaths of an Oklahoma family in Broken Arrow, Thursday, July 23, 2015.
Michael Bever's attorney, Rob Nigh, called the case a tragedy of immeasurable proportions.
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But, he said, 'a death penalty prosecution would result in his teenage sister being forced to recount and relive the brutal details of the carnage that her brothers wrought again and again'. They were brought in separately through different entrances to the courtroom.
A 911 call led Broken Arrow officers to a suburban home where they discovered the blood-splattered crime scene.
Parents April and David Bever, along with 12-year-old Daniel, 7-year-old Christopher and 5-year-old Victoria were found stabbed to death July 22, 2015. The sisters have since been placed in foster care, Kunzweiler has said.
Police said the brothers told them they wanted to be known as famous criminals and launch a nationwide killing spree.
'If he killed one person he was one person, that evened it out, ' Bentz said of Robert's motivations, while 'if he killed one more than one person, that would make him like a god'.
The older surviving sister did not testify at the February hearing, but a detective told the court that in an interview the girl stated that Robert and Michael had begun stockpiling body armor and knives, and her mother had "marked it up to 'That's just what boys do'". He said his main consideration was the two surviving siblings who lost everything and deserved the opportunity to move on with their lives. The sister also said she heard her parents discuss having perhaps been too rough on Robert and Michael when they were younger, according to the detective. Robert Bever pleaded guilty on all counts.