David Philip, 19, and Jake Sheehan, 20, torched the property in Mile End, east London, after a fight with another youth who lived there over a £15 debt.
At the end of the row Philip had shouted "we are going to burn your house down" - and went on to carry out his threat.
But only Shannen Vickers, 17, and her grandmother Pauline Adams, 57, were in the fifth floor flat that night in February.
Sheehan, of Bethnal Green, east London, and Philip, of Bow, east London, were both found guilty on two counts of murder by an Old Bailey jury.
There was loud cheering and applause from the public gallery as they were jailed today by Judge Richard Hawkins, and cries of "scum" and "you are going to rot" as they were taken down.
The judge said the two men were taking revenge for losing "street credibility" during the earlier confrontation.
He told the defendants that their "horrifying" crime had "struck at the heart" of family member Nicola Vickers, who lost both her mother and only child.
The court heard that Nicola had been warned of the threat earlier but had said: "Don't worry, no one could be so cruel."
"She had faith in the power of human nature," said the judge. "She was right. Such an act was a cruel act indeed."
The court heard that Philip owed £15 to the youth who lived at the flat but did not want to pay it back, and a fight broke out, which ended in the threat.
Simon Denison QC, prosecuting, said: "It is almost beyond belief, but an hour or so later Philip and Sheehan carried out their threat."
The court heard that the two killers siphoned petrol from a motorbike before setting fire to the vehicle.
They then poured the petrol through the letterbox in the flat and on to the door and set it alight, knowing it would be occupied, said Mr Denison.
"A major fire rapidly took hold. Thick black smoke and burning fumes filled the flat," he said.
"Pauline Adams and Shannen Vickers would have been asleep when the fire was set. They both woke up.
"Pauline went to Shannen's bedroom. Shannen was heard screaming for just a few seconds but she was not heard again.
"They were both overcome by the smoke and the fumes and they died together, lying side by side - two completely innocent people killed over a £15 debt and not losing face."
The court heard that a neighbour saw two people running down the stairs moments before the blaze was discovered, and that both defendants made phone calls to friends shortly afterwards, owning up to what they had done.
They were arrested later on the day of the fire, which took place at about 5.30am on a Sunday.
At the trial, both defendants tried to blame each other for starting the fire but the jury rejected their stories.
After the fire the victims' bodies were found in their night-clothes, lying side by side in Shannen's room.
It appeared that the teenager's grandmother had got up to help her but the smoke and fumes spread so quickly they were both overcome before they could leave the room. Post-mortems showed they had died from breathing in fumes.
The family's dog, Hooch, and their cat, Nut-Nut, also died in the blaze.
Nicola Vickers said in a family impact statement: "I cannot express in writing the pain of losing my daughter, she was my life."
She said Shannen, who she described as her "baby", was a "typical teenager who loved life" and had just finished a business studies course at Epping College, passing all her exams.
"Shannen was determined, hard-working, independent and caring, and I know she would have grown into a well-rounded young woman," she said.
"I would have loved to have seen my daughter get married and have children of her own. My chance to become a grandmother has been taken away from me."
She said both deaths had "left a large hole" in the lives of family members.
Her mother, she added, was a "true cockney lady" and "the life and soul of all our family".
"She was the person that we all went to for advice and help."
Detective Inspector Larry Smith said Sheehan and Philip had been "intent on taking their revenge" and threatened to start the fire when the dispute "came to a head".
"With callous disregard for life, they carried out that threat, setting fire to the house and taking the lives of a beloved grandmother and her granddaughter."
Comments