The teenager, who cannot be named because of her age, had claimed that mental illness diminished her responsibility for the killings of dinner lady Elizabeth Edwards and 13-year-old Katie Edwards.
A five-day trial heard the schoolgirl and her boyfriend planned the killings and carried them out “to the letter” when they were both aged 14.
The teenagers - thought to be Britain’s youngest ever couple to be convicted of murder - went on to share a bath, have sex, and watch four vampire-themed Twilight films after the murders 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 at Nottingham Crown Court - who heard that the defendants’ toxic Bonnie and Clyde-style relationship “led on to” the killings - took two-and-half hours to reject the girl’s defence and convict her of murder.
Her former boyfriend, who also cannot be named because of a court order, pleaded guilty to murder before the trial began.
At the start of the trial, 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 15-year-old 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.
Although she did not watch the subsequent killing of Katie, the girl described hearing “moaning” from the younger victim’s bedroom as the male defendant smothered her with a pillow.
Police found the bodies of Ms Edwards and her daughter on Friday April 15 this year when three officers forced their way into their house in Dawson Avenue, Spalding.
Post-mortem examinations found signs of defensive injuries on the older victim’s hands, suggesting she tried to fight off the boy, while her daughter was stabbed twice in the neck.
Defence psychiatrist Indranil Chakrabarti told jurors the teenager was suffering from an adjustment disorder and had not been acting rationally while plotting and helping to carry out the killings.
But forensic psychiatrist Philip Joseph said it was “blindingly obvious” that the girl was in a state of emotional turmoil rather than mentally ill.
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.
She told police the boy took “spare” clothes to the scene of the murders in a backpack, along with four knives.
In lengthy and matter-of-fact admissions to police, the schoolgirl said she and her boyfriend had planned the killings after a conversation which began with with a joke suggestion of committing a murder.
Explaining that failed attempts to put the plan into action had taken place on the previous two days, the girl told officers: “We went over the plan over and over again. It just kind of happened.”
After informing police that she witnessed Ms Edwards being killed, the girl told officers that she was shaking by the time her boyfriend entered the younger victim’s bedroom.
“I heard her say ‘I can’t breathe’ and ‘Get off me’. He came out of the room with blood on the side of his face, the side of his neck, his jumper and his hands as well,” the girl told two detectives.
After the killings, the girl said, her boyfriend “seemed fine” and they watched several films and took a bath for around 20 minutes, opting not to follow their plan and take their own lives.
Both killers face indefinite detention at Her Majesty’s pleasure - the juvenile equivalent of a life term for an adult.
The girl, wearing a dark cardigan, remained composed in the dock before being led away after the guilty verdicts.
Several minutes after the verdicts were returned, the girl began to dab at the side of her mouth with a white tissue and appeared to be stifling tears.
Mr Justice Haddon-Cave adjourned sentence on both defendants until next month and will also rule on whether or not they should be identified at a later hearing.
After ordering a pre-sentence report to be prepared on the girl, the judge told counsel: “I express the court’s gratitude for the exemplary way in which this case has been conducted.
“It is a wholly exceptional case by any standards and it has been investigated and prepared with exceptional skill and clarity.”