Safaa Boular, now 18, had started chatting online to fighter Naweed Husain when she was 16. She had decided to join him in Syria so they could marry, then carry out a suicide attack there while holding hands.
Husain had sent Boular's older sister, Rizlaine Boular, 3,000 pounds ($4,000) to pay for Safaa's travel arrangements, but the sisters were arrested in August 2016. They were released on bail but had their passport confiscated.
Safaa Boular continued chatting to Husain, and the pair discussed plans for her to attack the British Museum, one of central London's top attractions for visitors, with what she called "pineapples" - grenades.
"Safaa Boular's intention was to cause serious injury and death," said Sue Hemming of the Crown Prosecution Service.
After Husain was killed in Syria on April 4, 2017, Boular wrote that she wanted to be granted "martyrdom".
Instead, she was arrested eight days later, but her sister Rizlaine took on the planning of an attack on targets in central London, supported by the young women's mother Mina Dich.
The mother and daughter went on a reconnaissance visit to major landmarks in Westminster on April 25, 2017, and the following day they bought knives from a supermarket. They were arrested a day later.
Rizlaine Boular, 22, and Dich, 44, both pleaded guilty in February to planning acts of terrorism. Their pleas could not previously be reported because of the risk of prejudice to Safaa Boular's trial by jury.