Police allege 19-year-old Trent Thorburn had sex with his foster sister in the weeks leading up to her disappearance in October 2015.
Tiahleigh's body was found on the banks of the Pimpama River on the Gold Coast six days after she was last seen alive on October 30.
Thorburn's father Rick, 56, has been charged with her murder.
Rick Thorburn was taken to hospital shortly after being charged with his foster daughter's murder and is in an induced coma but his lawyer Kelly Thompson refused to comment on reports his condition was a result of self-harm.
During a bail application for Trent Thorburn at Beenleigh Magistrates Court on Wednesday, police prosecutor Elen French told the court despite repeatedly denying to police he'd had sex with Tiahleigh, a Facebook Messenger conversation with his cousin told a different story.
It's alleged Thorburn admitted he'd had sex with his foster sister in the conversation and expressed concern Tiahleigh might be pregnant.
Sgt French added Tiahleigh had confided to a friend "Trent tried to touch me" shortly before her disappearance.
It was also claimed Tiahleigh had informed her foster mother of Thorburn's alleged actions.
"It's not only corroborated that a sexual act had occurred but it's also corroborated he colluded with family members and lied in an attempt to avoid prosecution," Sgt French told the court.
"This defendant betrayed her trust. He abused his position as big brother and took advantage of her. A consequence of that is her young life has been taken."
Thorburn sat stony-faced throughout the hearing as a gallery packed with Tiahleigh's friends and family watched on.
Tiahleigh's mother Cindy Palmer was supported throughout by her partner, her eyes welling up as the allegations against her daughter's foster brother were detailed.
Despite Thorburn's lawyer John Ide arguing his client was young and had no criminal history, magistrate Pamela Dowse denied bail and the matter was adjourned until November 9.
Thorburn is facing four charges including incest, attempting to pervert the course of justice and two counts of perjury.
Rick Thorburn's matter has been adjourned until December 21.
His wife Julene, 54, and other son Josh, 20, are also facing charges of perjury and attempting to pervert the course of justice and will return to court on October 10.
