The Queensland foster family charged over the death of schoolgirl Tiahleigh Palmer is “very much divided” after two members agreed to appear as prosecution witnesses, their lawyer says.

The cases against Julene Thorburn, 54, and her son Josh, 20, were mentioned in the Beenleigh Magistrates Court on Monday morning after they were last month charged with perjury and attempting to pervert the course of justice.

The pair will plead guilty at their next appearance on November 21, their solicitor Jason Jacobson told the court in their absence.

Julene Thorburn’s partner Rick, 56, is in Brisbane’s Arthur Gorrie Correctional Centre after he was charged with Tiahleigh’s murder, while her other son Trent, 19, is also behind bars and faces an incest charge.

Outside court, Mr Jacobson said the charges against his two clients stemmed from their alleged failure to pass on “secret” information about Tiahleigh’s disappearance.

“Albeit late, they are now giving their complete co-operation to the police investigations,” he said.

“They’ve now taken the step of being prosecution witnesses to give evidence about everything they know, so you can take it from that that theirs very much is a family divided.”

Police previously revealed Julene Thorburn engaged a lawyer before her arrest but have declined to provide details about any deal she struck.

Tiahleigh’s biological mother Cindy Palmer, who attended the hearing flanked by several supporters wearing a “Justice For Tiahleigh” t-shirt, said the pair should have pleaded guilty a long time ago.

“They’ve had 11 months to plead guilty,” she said.

“They’ve been lying and being deceitful for the last 11 months, the least they should do is start being truthful now.”

The Beenleigh Magistrates Court last month heard Trent Thorburn allegedly admitted, over Facebook Messenger, that he had sex with 12-year-old Tiahleigh and was worried she may have been pregnant.

In the week after the arrests, detectives used an excavator to dig up the Thorburns’ Chambers Flat acreage and named the property as their “primary crime scene”.

The schoolgirl’s uniform and backpack were never found after her body was discovered on the banks of the Pimpama River last October.

Ms Palmer said she had been “up and down” over the last few weeks.

“(I’m) just trying to get through each day as it comes,” she said.

-AAP