The confession of Tiahleigh Palmer's foster brother to his mother that he'd had sex with the slain schoolgirl was a "catalyst" for her death, a court has heard.
Trent Thorburn, 20, was sentenced at the Beenleigh District Court on Thursday to four years' imprisonment after pleading guilty to two counts of perjury and one count each of incest and attempting to pervert the course of justice.
Having already served almost a year in pre-sentence custody, Thorburn's sentence will be suspended for five years on January 20, 2018.
The court heard Thorburn, then 18, had sex with Tiahleigh at least once before her death in late October 2015.
The 12-year-old's body was located on the banks of a northern Gold Coast river in November 2015, six days after police allege she was murdered by her foster father Richard Thorburn.
Crown prosecutor David Nardone said Thorburn, fearing Tiahleigh was pregnant, had confessed to his mother Julene he'd had sex with her.
"His confession to his mother was the catalyst for Tiahleigh's murder," Mr Nardone said.
The court heard Thorburn had also admitted his sexual relationship with Tiahleigh in a Facebook Messenger conversation with his cousin before confessing to his mother.
Following Tiahleigh's death, Thorburn repeatedly denied having had sex with her to police as well as denying he knew anything about her murder.
Thorburn then maintained those lies to a Crime and Corruption Commission (CCC) hearing in June 2016 before admitting his sexual relationship with his foster sister during another CCC hearing in September.
He did however continue to deny knowing anything about Tiahleigh's disappearance or death in that second hearing, Mr Nardone said.
During that second CCC hearing, Thorburn alleged Tiahleigh had initiated the sexual act and he was unable to resist her advances due to being under the influence of alcohol, a proposition Judge Craig Chowdhury dismissed as "completely implausible".
Thorburn's mother Julene Thorburn is due to face sentencing on charges of perjury and attempting to pervert the course of justice in November.