The teenager, who cannot be named because of her age, had claimed a mental disorder diminished her responsibility for the killings of dinner lady Elizabeth Edwards and 13-year-old Katie Edwards.
But a five-day trial at Nottingham Crown Court heard that the schoolgirl was not suffering from a mental illness when she and her boyfriend planned the killings “to the letter” when they were both aged 14.
The girl, who remained composed in the dock, and her boyfriend, also now 15, are thought to be the youngest defendants to be convicted of double murder in a British court.
After the verdicts had been delivered, Mr Justice Haddon-Cave said it had been an “exceptional case by any standards” and praised the quiet dignity of family members who sat through the trial.
Jurors were told that the sweethearts – also believed to be the youngest couple to be convicted of murder – shared a bath, had sex, and watched four vampire-themed Twilight films after the killings in Spalding, Lincolnshire, last April.
The female defendant admitted manslaughter but denied murder, claiming to be suffering an abnormality of mental function which impaired her ability to form rational judgments.
But jurors – who heard that the defendants’ toxic Bonnie and Clyde-style relationship “led on to” the killings – took two-and-a-half hours to reject the girl’s defence and convict her of murder.
Her boyfriend, who stabbed and smothered the victims as they slept, pleaded guilty to murder before the trial began.
At the start of the hearing, prosecutor Peter Joyce QC said the victims were stabbed a total of 10 times in a “cold, calculated and callous” pre-planned attack at their home.
In police interviews a day after her arrest, the girl said her boyfriend had knifed Ms Edwards through the voice box to ensure her daughter was not woken by screams or cries for help.
Police found the bodies of Ms Edwards, who was believed to be originally from Edinburgh, and her daughter on 15 April when three officers forced their way into their house in Spalding.
Post-mortem examinations found signs of defensive injuries on the older victim’s hands, suggesting she had tried to fight off the boy, while her daughter had been stabbed twice in the neck.
In police interviews and assessments with psychiatrists, the female defendant described the killings as “a breeze” and gave a horrifying account of blood being spattered on a wall and her boyfriend’s face and hands.