The oldest daughter was 12 years old. That is old enough to be left in charge of her sisters until mom gets back. My 2 oldest daughters, at age 10, baby sat for families all the time.
They took a 40 hour course in baby sitting, trained and certified in First Aid and CPR. The training was by the Borough Parks and Rec and Fire Department in Ketchikan. 
 OCS here at times has told people that 10 years old is safe for kids to be left alone. Our Borough public bus allows kids to ride the bus by themselves at age 8. School district here allows kids 5 and up to walk to and wait at bus stops. Many people do and have left their kids with older siblings while they run quick errands.  My parents did and I did too. 
How do you know if there were working smoke alarms or not ? In a fire that intense they melt. 
The parents were taking responsibility and good care of their children by all accounts of neighbors.They both had jobs and were scheduled to be moving out of the trailer into better housing.