Francesca Bimpson suffered massive burns when Graham Heaps, 44, started the fire last December because he was bitter about a break-up with her aunt.
Heaps took revenge on the family of the woman who rejected him by torching their home as they slept.
Francesca was trapped inside the blazing property, suffering 86% burns to her tiny body.
On the night of the attack Francesca and her brother and sisters had been happy and excited after getting a Christmas tree at the family home in Liverpool.
But Heaps, 44, who had a record for petty crime and burglary stretching back 30 years, could not handle rejection after the breakdown of his relationship with her aunt, Liverpool Crown Court heard.
While the family of six were in bed, Heaps poured petrol through the letterbox before starting a blaze which ripped through the semi-detached house in Everton.
Francesca died in agony in hospital three weeks after the attack on December 1 last year.
Her mother, Eleanor Skelhorne, 37, suffered minor injuries and smoke inhalation and her father, Kieron Bimpson, also 37, suffered leg injuries jumping from a first-floor window.
Francesca’s sisters, Anne-Marie, then aged 17 months, Christina, eight, and 14-year-old brother, Kieron, were treated for the effects of smoke.
Heaps was convicted of murder and sentenced to a mandatory life term in jail.
Outside court, Francesca’s father said: “There will never be a fitting justice for our baby. Her life was cruelly and prematurely taken by a coward.”