Safaa Boular was aged 17 when she allegedly schemed with her sister to launch assaults in the British capital in April, 2017.
She began plotting after failing the year before to join her then-fiance Naweed Hussain, a member of the ISIS subsequently killed in Syria, the court heard.
While in custody for allegedly trying to travel to ISIS territory, she encouraged her elder sister Rizlaine Boular to carry on with the plot, jurors were told.
In calls from jail, she talked with her 21-year-old sibling about arranging an "Alice In Wonderland" themed tea party, which prosecutors said was code for an attack.
The duo also referenced a "Mad Hatter" and "pineapples", other code words in the plot, they added.
During their investigation members of the security services posed as ISIS fighters online to engage with the pair and track their activities.
Prosecutors said the sisters and their mother Mina Dich, 43, had carried out reconnaissance around major landmarks in Westminster and purchased a pack of knives and a rucksack.
The trio were arrested on April 27 last year in raids by counter-terrorism police, during which Rizlaine Boular was shot.
She has since pleaded guilty to terrorism offences.
Safaa Boular, who lived at home with her mother in southwest London, has denied two counts of preparing acts of terrorism.
Her defence lawyer Joel Bennathan told jurors that she had been "groomed" by Hussain, who was twice her age, and that her family had "encouraged" and "celebrated" it.
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