Safaa Boular, 18, plotted the grenade and gun attack in her home town after the authorities intervened to stop her travelling to Syria to marry an ISIS terrorist she had met online.
She is the youngest female to be charged with planning an IS attack in Britain.
Boular was only 16 when she made contact with British-born ISIS terrorist Naweed Hussain, 32, discussing marriage and how they would don his-and-hers suicide belts.
But her hopes of joining him were dashed when she was stopped at the airport in August 2016 following a family trip to Morocco, and her passport was confiscated.
Instead Boular decided to plan an attack in Britain, detailing it in coded language -- grenades were "pineapples" -- to online contacts who were in fact undercover agents.
Hussain was later killed in a drone strike.
When Boular was charged with preparing terrorist acts in April 2017, she passed the baton to her sister Rizlaine, 22, and their mother Mina Dich, 44, who hatched their own plan.
The trio were taped talking about an Alice in Wonderland-themed tea party, which the prosecution argued was code for an attack.
They pleaded guilty to terror offences, along with a fourth woman, Rizlaine's friend Khawla Barghouthi, 21, who later admitted failing to alert the authorities.
Safaa Boular denied the plot but was found guilty on Monday after a trial.
Dean Haydon, Britain's top counter-terror police officer, said the plot "involved a family with murderous intent, the first all-female terrorist plot in the UK connected to Daesh (ISIS)".
He added: "All three women were filled with hate and toxic ideology and were determined to carry out a terrorist attack.
"Had they been successful, it could well have resulted in people being killed or seriously injured."
Rizlaine, her mother and their friend will be sentenced on June 15, although a date has not yet been set for Safaa Boular's sentencing.
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