The house in suburban Harristown in Toowoomba, about 130km west of Brisbane in the Darling Downs region, went up in flames in the early hours of Wednesday.
Six people, including two adults and four children who are believed to be related, escaped the blaze.Â
Two of the children - girls aged four and seven - suffered critical burns and died at Queensland Children's Hospital overnight after being airlifted to Brisbane following the blaze.
A body, believed to be that of a nine-year-old boy, was found in charred remains of the home on Wednesday, although police say he has yet to be formally identified.
A neighbour said a resident of the home repeatedly yelled a name after escaping the fire before the body was found.
The home remains a crime scene as police investigate the fire after homicide detectives were called in.
"Police are continuing to treat the fire as suspicious, are not looking for anyone else in connection to this incident at this time," Queensland Police said on Thursday.
A 36-year-old woman remains in a critical condition at Royal Brisbane Women's Hospital under police guard.
Of the two men who were taken to a Toowoomba hospital, one, aged 34, is in a stable condition with serious injuries.
The other, an 18-year-old, is believed to have a hand injury.
An 11-year-old boy managed to escape the fire and was uninjured.
"This is a tragic event and we are deeply saddened by this incident," Detective Inspector Renee Garske said on Wednesday.
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