The former foster mother and brother of Logan schoolgirl Tiahleigh Palmer, whose body was found on a Gold Coast riverbank, intend to plead guilty when their matter returns to court next month, their lawyer says.

Julene and Joshua Thorburn, 20, were last month charged with perjury and attempting to pervert the course of justice in relation to the police investigation into the 12-year-old's death.

Tiahleigh disappeared in October last year and her body was discovered on a riverbank at Pimpama about a week later.

Rick Thorburn is yet to face court over charges of murder, and his other son, 19-year-old Trent, is facing offences including incest.

Lawyer Jason Jacobson, representing Julene and Joshua Thorburn, appeared in court today and said his clients would be pleading guilty.

Mr Jacobson said the pair had agreed to testify as witnesses in Rick Thorburn's court case.

"The charges against Joshua and Julene Thorburn stem from them not revealing information — information that was shared with them," Mr Jacobson said.

"Albeit late, they are now giving their full cooperation to the police investigations and what's more, they've now taken the step of agreeing to be prosecution witnesses and to give evidence about everything they know.

"So you can take it from that that theirs is very much a family divided."