The foster brother who had sex with Queensland schoolgirl Tiahleigh Palmer before her alleged murder has been sentenced to four years in jail for incest, perjury and attempting to pervert the course of justice.

Trent Thorburn, 20, pleaded guilty at the hearing today at the Beenleigh District Court, south of Brisbane.

Tiahleigh's body was discovered on the banks of the Pimpama River on the northern end of the Gold Coast on November 5, 2015.

Her foster father, Rick Thorburn, is awaiting his trial for the alleged murder of the 12-year-old.

Rick Thorburn had reported the girl missing six days earlier and claimed he last saw her alive while dropping her at school.

Judge Craig Chowdhury said at today's sentencing hearing that Trent Thorburn took part in "a sustained, immoral and disgraceful plan" with other family members to cover up Tiahleigh's death.

Trent Thorburn has already spent nearly a year in custody and will be released in late January 2018, after serving 16 months.

Judge Chowdhury chastised Thorburn for lying to police and authorities, and not revealing everything about Tiahleigh's disappearance.

"You needed to man up and be honest and be truthful and your failure to do so is a shocking reflection on you," Judge Chowdhury said.

"You've got no-one to blame but yourself for the situation you find yourself in."

Brother's confession 'catalyst' for Tiahleigh's death

Tiahleigh's body was discovered on the banks of the Pimpama River in 2015. ( Supplied: Queensland Police Service )

Judge Chowdhury said when Trent Thorburn revealed to his mother he had had sex with his 12-year-old foster sister that "became the catalyst for her tragic death".

The judge told the hearing he "couldn't get his head around" how someone with such good prospects could end up in this situation.

Prosecutor David Nardone told the court earlier today Thorburn had admitted in a Facebook conversation with his cousin to having sex with his foster sister.

Further admissions were captured on secret surveillance devices in the family home, and by an undercover police officer who shared a cell in custody with Thorburn.

Mr Nardone told the court Thorburn lied repeatedly to police and the Crime and Corruption Commission about having sex with Tiahleigh, and about whether he knew his foster sister had been murdered.

"The lies told by Mr Trent Thorburn were no spur-of-the-moment decision," Mr Nardone said.

Judge Chowdhury told the hearing Thorburn's initial explanation that Tiahleigh had blackmailed him into having unprotected sex, or else she would hurt his dog, sounded "completely implausible".

Trent Thorburn will be released in late January 2018, after serving 16 months in jail. ( AAP: Travis D. Hendrix )

The court heard Thorburn had concerns his foster sister was pregnant, because she had been complaining about stomach pains.

"It seems that this fear of pregnancy triggered this extraordinary situation," Judge Chowdhury said.

Thorburn's lawyer Robert East had told the court there was no excuse for having sex with a minor, but argued his client had nothing to do with Tiahleigh's death.

Mr East told the court Thorburn was expected to face criminal charges for the relationship with his foster sister, but was horrified about the subsequent events surrounding Tiahleigh's death.

He said his client "acted out of misguided loyalty to his family".

Tiahleigh's mother welcomes 'justice'

Sorry, this video has expired Cindy Palmer addresses the media

Tiahleigh's biological mother, Cindy Palmer, said: "Obviously everything we heard in court today was hard to hear.

"We all know that Trent had ample opportunity to come forward with what he knew.

"This is just the second step in justice for Tiahleigh."

In July, Thorburn's younger brother Joshua Thorburn was jailed for a minimum of three months for perjury and perverting the course of justice.

Their mother, Julene Thorburn, has pleaded guilty to the same offences and will be sentenced in November.