The twins' co-defendant, 14-year-old Hakima Rijlane, was acquitted on the same charge by judges at the juvenile court in Rabat on Tuesday.
The three girls had been arrested earlier this month and were accused of plotting with 18 adult accomplices to carry out attacks in a Rabat supermarket and against the royal family.
Prosecutors had called for maximum penalties in the trial of the twins, named by a local newspaper as Imane and Sanae al-Ghariss.
The twins are also accused of contemplating a plot against Morocco’s King Muhammad VI.
The three girls were tried in a juvenile court, separately from their alleged adult accomplices.
The juveniles will likely be sent to a young offenders’ institution, then to an adult prison when they reached 18, to serve out the rest of their sentence.
The defence pleaded for leniency in light of what they said was the difficult social environment in which the girls had grown up and the alleged exploitation by the girls’ suspected adult accomplices of their “youth and naivete”.
The twins’ mother, Rashida T’Riae, accused one of the girls’ alleged accomplices, a plumber named Abd al-Kadar Labsir, of having influenced the twins.
In Monday’s closed-door hearing, the twins admitted to five juvenile court judges that they had planned to blow themselves up in public places, said defence lawyer Saad Guennou.
One of them planned to explode themselves up in the wines and spirits section of the a supermarket.
The third accused girl, Hakima Rijlane, had denied any involvement in the plans.
Abd al-Haq, father of Hakima, who
was acquitted on Tuesday
The twins’ mother on Monday thanked Muslim cleric Rashid Nafae, who preaches at a mosque in the poor neighbourhood where the twins live, and to whom the girls allegedly wrote to ask if carrying out a suicide attack would be in line with Islamic teachings.
Nafae replied it would not and informed the authorities who arrested them in early September, days before the girls planned to carry out the attacks, said police.
“If the preacher had not denounced them, there would have been a tragedy,” said their mother.
A local newspaper said that the twins had dabbled in begging and prostitution in Rabat before being “fascinated” by the 11 September 2001 attacks in the United States.
In May, Morocco was rocked by the first human bombing attacks in the kingdom, when five bombs went off simultaneously in the economic hub of Casablanca.
Forty-five people died as a result of the attacks, including 12 suspected bombers.