The claimed incestuous relationship that allegedly led to Tiahleigh Palmer's murder began with a crush, the slain schoolgirl's grandmother has said.

The claim comes as police reveal details of the toxic relationship between the pair and how Tiahleigh's foster brother Trent Thorburn, 19, allegedly took advantage of the girl, whose biological mother, Cindy Palmer, is a Kiwi.

In an interview with the Courier-Mail, Sue Palmer, who cared for 12-year-old Tiahleigh until she was 10, said her granddaughter had the opportunity to leave the Thorburn family's rural home, but insisted on remaining at the idyllic Chambers Flat property.

"She said she wanted to stay because of the boy and because she loved the horses on the property that they have there," Palmer told the Courier-Mail.

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Tiahleigh's backpack and school uniform have never been found. Photo / News Corp Australia
Tiahleigh's backpack and school uniform have never been found. Photo / News Corp Australia

"We didn't have any idea as to what was actually happening there - just that she had a crush on the boy and that was the main reason she wanted to stay."

Weeks after the conversation between the grandmother and granddaughter, Tiahleigh was murdered in October last year, allegedly by her foster father, Rick Thorburn.

Her body was dumped and found decaying on the banks of the Gold Coast's Pimpama River by a trio of shocked fisherman a week later.

Police allege the foster father murdered the schoolgirl in his car after discovering his son Trent had allegedly sexually abused her.

Trent has been charged with incest, perjury and attempting to pervert the course of justice and was denied bail. His father has been charged with murder and interfering with a corpse. He reportedly swallowed pills while in police custody following his arrest on Tuesday, and remains in an induced coma. He is scheduled to appear in court in December.

In court yesterday, where the teenager applied for bail, police prosecutor Sergeant Ellen French asserted details of the toxic relationship between the foster siblings that led to her death.

French said that despite repeatedly denying any sexual relationship with Tiahleigh, police had received a statement from his cousin saying he had told her he had sex with his foster sister, AAP reported from Beenleigh Magistrates Court.

"She said he made the disclosure on Facebook messenger and that she had saved a copy of the conversation, which was subsequently provided to police," she told the court.

Trent Thorburn, 19, was allegedly involved in a sexually abusive relationship with his foster sister, Tiahleigh Palmer. Photo / Facebook
Trent Thorburn, 19, was allegedly involved in a sexually abusive relationship with his foster sister, Tiahleigh Palmer. Photo / Facebook

"During the conversation, he says he had sex with the victim on the prior Monday and he had only done so because she had threatened to kill his dog."

French said Thorburn, 19, had betrayed the girl's trust.

"He abused his position as big brother and took advantage of her. A consequence of that is her young life has been taken," she said.

Police also allege Tiahleigh herself revealed Thorburn's criminal behaviour in a conversation with a friend shortly before her disappearance.

Tiahleigh said that "when her foster carer went out, their son would then have sex with her", according to the friend.

Detective Inspector Damien Hansen described the murder investigation as the most difficult of his career. Photo / Jack Tran, News Corp Australia
Detective Inspector Damien Hansen described the murder investigation as the most difficult of his career. Photo / Jack Tran, News Corp Australia

Tiahleigh had also said "Trent tried to touch me" to her friend.

Furthermore, French claimed Tiahleigh had informed her foster mother Julene Thorburn about the sex but the 54-year-old had taken no action against her son.

Trent Thorburn was yesterday denied bail and remains in police custody over the serious charges.

According to the Courier-Mail, police will allege Rick Thorburn most likely smothered Tiahleigh on October 29.

The blue Ford Falcon described by police also week as a 'vehicle of interest' in the murder investigation. Photo / Marc Robertson, News Corp Australia
The blue Ford Falcon described by police also week as a 'vehicle of interest' in the murder investigation. Photo / Marc Robertson, News Corp Australia

The foster father initially said he was the last person to see the 12-year-old alive when she went to school the following day, October 30.

This information was included in a missing persons notice that went out six days later, but police now believe Tiahleigh never made it to school that day.

While Trent and Rick Thorburn remain in police custody, Julene and Josh Thorburn, 20, have also been charged with perjury and perverting the course of justice. Both are due to face court next month.

The allegations against the family have also raised questions around the selection of carers in Queensland.

Child Safety Minister Shannon Fentiman revealed yesterday that the Thorburns had been approved as foster carers in August 2014.

Tiahleigh was in their care for 10 months.

Four weeks after her death, the foster father published a post on Facebook advertising accommodation for one or two people in their "happy home environment on acreage".

The family had also operated a day-care centre, Fentiman said, before police contacted authorities in April.

Their day-care approval was then revoked and foster-care approval suspended, Fentiman said.