Tiahleigh 'Tia' Palmer was murdered by her foster father to protect his 19-year-old son who had sex with the 12-year-old schoolgirl days before her death and feared she was pregnant, police allege.

The foster father, 56-year-old Richard Thorburn, is now in an induced coma in a Brisbane hospital after collapsing while in police custody. Police have refused to formally confirm reports he swallowed pills. He is charged with one count each of murder and interfering with a corpse.

Meanwhile, his teenage son Trent is behind bars charged with incest, attempting to pervert the course of justice and perjury.

New allegations in the increasingly disturbing case emerged in a Queensland courtroom on Wednesday, as police prosecutors referred to a series of Facebook messages allegedly revealing Trent feared he had impregnated his young foster sister.

The court heard Trent confided in his cousin via Facebook that he had sex with Tia only four days before she went missing.

The 19-year-old allegedly said he had sex with Tia because she had threatened to kill his dog, and that he was concerned she could be pregnant.

Police pounced on the Thorburn family on Tuesday, bringing to an end a marathon investigation involving up to 50 detectives.

It is alleged the family all lied to authorities to cover up Tia's death.

Richard's older son Joshua, 20, and wife Julene, 54, were both charged on Tuesday night with attempting to pervert the course of justice and perjury, but released without facing court.

Tia's badly decomposed body was found by fishermen on the banks of a river on Queensland's Gold Coast in November 2015, just hours after a missing person's alert was issued.

The 12-year-old lived with her foster family for 10 months at their two-hectare property in Chambers Flat - a rural pocket of strawberry farms and horse paddocks in the Logan area south of Brisbane.

Richard - a former truck driver and muscle-car enthusiast - operated a food truck, which he named Nothing Healthy Here. With his two sons, he drove to festivals and other events around Logan selling hot dogs, hot jam doughnuts, churros and pancakes from the van.

While she lived with the Thorburns, Tia attended Marsden State High School.

Police allege Tia was killed at the family's four-bedroom, brick veneer home on October 29, before her body was dumped 30 kilometres away in the Pimpama River.

Trent was denied bail in Beenleigh Magistrates Court on Wednesday, despite his lawyer arguing his age and lack of criminal history were mitigating factors.

Police prosecutor Sergeant Ellen French told the court Trent had denied any involvement or sexual conduct with Tia on five separate occasions, before eventually confessing he had "lied on multiple occasions in order to conceal the sexual intercourse".

"The victim was put into the care and protection of this defendant and his family but rather than providing her with the care and love that she required as a child, this defendant betrayed her trust," Sergeant French said.

"He abused his position as a big brother and took advantage of her, and the consequences are a young life has been taken."

A friend of Tia's told police the young girl had approached her about a month before her alleged murder, saying "Trent tried to touch me", Sergeant Ellen said.

"She further said that the victim claimed to have disclosed this to her foster mother," she said.

A sigh of relief and a stifled cry of "yes" from the crowded courtroom greeted the refusal of bail, with Tia's biological mother Cindy Palmer echoing that relief outside, flanked by supporters.

Among them were Tia's former foster father Wayne Pemberton and his son Cameron, who paid tribute to the "good girl".

Wayne said the "feisty" 12-year-old lived with them for two and a half years and described his "total shock" at hearing the man who cared for Tia after him had been accused of her murder.

"You don't expect that from a carer," he said.

"That's not what you're here to do.

"To hear that and for everyone in the family to [allegedly] be involved it's just unthinkable."