Safaa Boular, 18, was convicted of preparing terrorism acts at home after failing to travel to Syria to join militants of the so-called Islamic State.
A British teenager was sentenced to life in prison on Friday as the youngest woman to be convicted of plotting a terror attack on British soil, which was prevented last year by a joint investigation by counter-terrorism police and the security services.
Safaa Boular, 18, of Vauxhall, London, was convicted of preparing acts of terrorism and attempting to travel to Syria to join the so called 'Islamic State' militants.
The teenager was part of the UK's first all-female so-called Islamic State cell, alongside her mother and sister, who were jailed in June. The judge alleged she was an ongoing threat with "deeply entrenched" views.
Boular had been groomed online for terrorist activity by her fiance, Naweed Hussain, a so-called Islamic State fighter, who later died, prosecutors said.
The trial heard that she had wanted to join Hussain in Syria but could not travel as she had already fallen under the suspicion of the security services.
Hussain urged her to attack the UK — and gave away his plans to MI5 officers posing online as extremists.
When he was killed in what is believed to have been a targeted drone strike, Boular pledged to go on without him.
Deputy Assistant Commissioner Dean Haydon, senior national coordinator for Counter Terrorism Policing, said: "This investigation started with Safaa and her attempts to travel out to Syria, marry a Daesh fighter and support their terrorist activity.
"Having been prevented from travelling to Syria, she then set about plotting an attack in the UK but her plans were being covered by the counter terrorism network and security services."
Boular revealed his plans for a grenade and gun attack in London to another MI5 undercover officer who contacted her online, pretending to be Hussain's commander.
Following her trial at the Old Bailey earlier this year, a jury convicted Boular of two counts of preparing acts of terrorism: planning to die in a suicide bombing in Syria and preparing to attack the British Museum when travel was not possible.
Boular at the trial declared herself deradicalised. Her defence team said she had been groomed and exploited.
Her radicalisation began six years ago, when her mother began apparently to support jihadist causes.
Two years later her older sister, Rizlaine, attempted and failed to reach Syria.
After being charged and held on remand for her suspected travel plans, prosecutors alleged Boular had encouraged Rizlaine from prison to take her place, referring to the attack in code as a "Mad Hatter's tea party" and weapons as "cakes".
During the sentencing hearing, Safaa Boular's defence team urged the judge to take into account what they described as her deeply troubling background, including the influence of her "very radicalised sister" and "neglectful mother".
Her sister Rizlaine Boular, 22, was sentenced to life, two months ago, with a minimum term of 16 years.
Mina Dich, their mother, was jailed for six years and nine months for assisting the plans.
The pair had admitted preparing a terror attack in Westminster by carrying out an inspection of well-known landmarks and buying knives and a rucksack.