Emergency services were called to a home on Todman Street just after 6am on Sunday to find the two-storey house fully engulfed, with two neighbouring properties also alight.
Queensland Police confirmed late on Sunday five brothers - aged 11, 10, four-year-old twins and a three-year-old - and their 34-year-old father died in the blaze and their bodies had been recovered.
A 21-year-old woman thought to have been inside the house when the fire broke out also managed to escape.
The 28-year-old woman believed to be the children's mother was treated for smoke inhalation but was otherwise physically unharmed.
"This poor lady has lost her entire family," Queensland Police Superintendent Mat Kelly told reporters.
"She's terribly emotionally distressed at the moment but doesn't have any actual injuries."
He said many in the small community would be affected by the fire, including emergency services on scene who knew the victims well.
"(It) is truly tragic as these young boys could have become men into the future," Mr Kelly said.
At least 30 fire personnel remained at the site on Sunday afternoon as investigations began, with drones sent overhead to assess the wreckage.
Queensland Fire and Emergency Services Assistant Commissioner John Cawcutt said the whole structure was unstable, with embers still smouldering within the debris.
He said the two damaged properties on either side of the destroyed house had been checked but crews would remain on site to secure the smouldering home so more fires did not break out.
"So that's one of the reasons we will be there for the next few days," he said.
"As you start to lift the roof off the structure, then small fires often crop up again.
"There's still quite a lot of smoke, and there is fire still burning in parts of the property."
Mr Cawcutt said the blaze was "one of the worst fires we've had for a long time".
"When you get a tragedy like this, it touches many people," he said.
Nine people were treated at the scene for smoke inhalation and minor injuries and two were taken to hospital in a stable condition.
Eight-kilometre-long Russell Island lies between the Queensland mainland and North Stradbroke Island and has a population of about 3700.
Queensland Police Commissioner Katarina Carroll thanked the "brave" first responders for their efforts at what could only be described as a devastating scene.
Queensland Premier Annastacia Palaszczuk and others shared sympathies for those involved in the tragedy.
"My thoughts are with the whole Russell Island community at this very sad and tragic time," she tweeted.
"The early reports are heartbreaking and this tight-knit community will need our support in the days ahead," Queensland Opposition Leader David Crisafulli posted on social media.
Redland City Mayor Karen Williams described the fire as a dreadful tragedy.
"We are aware the impact of this incident will have deep and lasting ramifications," she told AAP in a statement.
"The Southern Moreton Bay Islands are an isolated community and will need all our support as the community moves through and heals from this terrible situation."
She said the council was working closely with the responding agencies, including Community Champions and Red Cross, to help find accommodation for a family from one of the homes.
Police are not treating the fire as suspicious.
Local authorities are in the process of setting up a fundraising account through the local registered charitable trust, The Redland Foundation.
