More people are expected to be arrested over the 2004 murder of Brisbane's April and Ian Bailey after a woman was taken into custody.
The 34-year-old woman arrested on Saturday morning is the first person apprehended over the siblings' cold case deaths.
The pair are believed to have been murdered before their Deception Bay home was set on fire on May 28, 2004.
Ian, 29, had been caring for his 31-year-old sister, who was recovering from schizophrenia and working as a beautician at the time of her death.
Ms Bailey's daughter Violet was four years old when she witnessed the murders and was discovered uninjured at the scene by police.
Detective Inspector Damien Hansen said the arrested woman was an associate of the siblings and part of the alleged motive for the homicides was drug related.
She was identified after police used new technology to analyse triple-zero calls made before the house fire, in which Mr Bailey is believed to be heard yelling.
Police are expected to charge her with two counts of murder and arson.
Det Insp Hansen said he expected more people would be arrested for murder as the investigation continues.
He said the investigation had been helped by a number of people who had not previously given evidence but came forward after police launched a campaign for information last year.
"They've changed their lifestyles, their morals, and that has helped us greatly in solving this," he said.
A $250,000 reward and the promise of indemnity still stands for anyone with information regarding the homicides.