Two-year-old Harley Cheenanow and his 18-month-old sister, Haley, died after the fire broke out at their grandmother's house.
The volunteer fire department in nearby Loon Lake was called but didn't respond, because service to the reserve had been cut weeks earlier over unpaid bills.
Investigators said Wednesday the source of the fire can't be determined because of the "complete burn" nature of the blaze.
"(We) have confirmed that a battery-operated smoke detector was in the home at the time of the fire, but it is not known if it was in normal working condition," said a statement from Emergency Management and Fire Services.
RCMP officers were the only first responders that showed up to help as the fire raged.
The grandmother managed to get out alive but the children, carried out of the burning home by their father, died at the scene.
Chief Richard Ben of the Makwa Sahgaiehcan First Nation has said his reserve has a working fire truck, but they don't have enough money for proper equipment or to train crews to use it.
Perry Bellegarde, national chief of the Assembly of First Nations, said in a letter to the federal government last week that First Nations receive insufficient funding to ensure a safe and healthy environment.
He said the situation has reached a "critical level" and asked for a meeting with Aboriginal Affairs Minister Bernard Valcourt.
Valcourt has said Makwa Sahgaiehcan, like all other reserves, gets sufficient funding for fire services and it's up to band officials to decide how the money is spent.
In his letter, Bellegarde also said the government policy that caps annual funding increases for First Nations needs to change.
"The lifting of the two per cent cap and replacing it with an appropriate escalator is a good place to start," he wrote.
Valcourt's office has indicated the minister has agreed to meet with Bellegarde.
(The Canadian Press, CKOM)