The moment Tiahleigh Palmer's foster brother confessed to having sex with the 12-year-old, the Brisbane schoolgirl's fate was sealed, a court has heard.
Trent Thorburn, 20, confessed to his mother in late October 2015 he'd had sex with Tiahleigh amid fears the young girl was pregnant.
Within hours it is alleged Tiahleigh was murdered.
On Thursday, Thorburn was given a head sentence of four years' imprisonment after pleading guilty to four charges including incest at the Beenleigh District Court.
With time already served, Thorburn's sentence will be suspended for five years on January 20, 2018.
The court heard Thorburn had sex with Tiahleigh on at least one occasion in October.
Concerned she was pregnant, Thorburn revealed what had happened in a Facebook messenger conversation with his cousin.
Following his cousin's advice, Thorburn then confessed to his mother, an action crown prosecutor David Nardone said was the "catalyst" to Tiahleigh's death.
Judge Craig Chowdhury said Thorburn engaged in "a sustained, immoral and disgraceful plan" with other family members to cover up Tiahleigh's death.
Thorburn's barrister Robert East said his client never expected Tiahleigh to be killed and had described "waking up the next morning hoping it was all a bad dream but it became a living nightmare".
Mr East said his client had lied out of "misguided loyalty" to his family and would have to live with the knowledge his misconduct led to "an innocent life being lost".
After Tiahleigh's body was discovered on the banks of the Pimpama River in November 2015, Thorburn denied having sex with her to police on multiple occasions.
He also made false statements and testimony at two Crime and Corruption Commission hearings into his knowledge of Tiahleigh's death.
Judge Chowdhury said Thorburn had failed in his obligation to do the right thing.
"You needed to man up and your failure to do so is a shocking reflection on you," Judge Chowdhury said.
Tiahleigh's mother Cindy Palmer, who was in the court for the sentencing, said Thursday's outcome was just the second step in the quest for justice for her daughter.
Thorburn's mother is due to be sentenced on charges of perjury and attempting to pervert the course of justice in November. Thorburn's father Richard has been committed to stand trial for murder at a date to be determined.
