Julene and Joshua Thorburn will testify against Tiahleigh Palmer's accused killer Rick Thorburn, according to their lawyer.
The dramatic development was revealed outside Beenleigh Magistrates Court on Monday, after Julene, 54, and Joshua, 20, were represented in court on charges relating to the alleged murder of the 12-year-old foster child in October last year.
The duo, who did not attend court, face one count each of perjury and attempting to pervert the course of justice during the murder investigation, which ultimately led to Rick Thorburn, 56, being charged with Tiahleigh's murder.
In court, Gold Coast lawyer Jason Jacobson said his clients intended to plead guilty to all charges.
While speaking to media outside court, Mr Jacobson said Julene and Joshua's charges stemmed from information they were ordered to keep quiet by Rick Thorburn.
He said his clients had made full admissions to police and were prepared to assist the prosecution as witnesses when Rick Thorburn eventually faced court.
"Albeit late, they are now giving their complete co-operation to the police investigation, and what's more is that they now have taken the step of agreeing to be prosecution witnesses and will give evidence about everything they know," he said.
"You can take it from that their's is very much a family divided."
The duo's bail was extended until their next court appearance.
Murder-accused Rick Thorburn claimed he dropped Tiahleigh off at Marsden State High School on the morning of October 30, 2015.
The 12-year-old's decomposing body was found on the banks of the Pimpama River six days later.
After an almost 11-month investigation, police charged Mr Thorburn on September 20 with one count of murder and one count of interfering with a corpse.
Police will allege that Rick Thorburn killed Tiahleigh in a bid to cover up the alleged crimes of his son Trent.
Trent Thorburn, 19, has been charged with incest, perjury and attempting to pervert the course of justice.
He faced court on September 21 and was remanded in custody until November.
Tiahleigh's biological mother Cindy Palmer was in court for the hearing.
Speaking to media, Ms Palmer said she was satisfied with the guilty plea but was critical of the duo for allegedly lying to police.
"They are being charged with a lot less than they need to be, but hopefully they get what they deserve," she said.
"They've been lying and being deceitful for the last eleven months, so the least they can do is start being truthful now."
Ms Palmer said Queensland's foster care system needed "a lot of work."
"There's so many aspects of it that need working on and it's not going to be an easy fix," she said.
"They are just kidding themselves if they think it is."