A jury found that Safaa Boular plotted with her partner, Naweed Hussain, an Islamic State militant, to launch a grenade and bomb attack on the British Museum in Bloomsbury, central London.
She was also found guilty of an attempt to travel to Syria to join IS. She was the final member of Britain’s first all-female terror cell to be sentenced at the Old Bailey, after the convictions of her mother, Mina Dich, 44, older sister, Rizlaine Boular, 22, and family friend Khawla Barghouthi, 21.
The court heard that Boular met Hussain, from Coventry, who was 30 and a known IS recruiter, online when she was 16.
They were in contact for three months before they declared their love for each other and had what she regarded as an online Islamic marriage.
Duncan Atkinson, the prosecutor, told jurors that Boular wanted to marry Hussain and carry out a suicide attack in Syria.
When police prevented her from joining him in the country, messages on her phone revealed repeated conversations about a potential attack in the UK.
Boular claimed she never agreed to any attack. Her defence lawyer, Joel Bennathan, said she was a child when Hussain groomed her. “Around November  he proposed to me about an attack at Christmas,” Boular told the court. “He asked me if I was scared of being in an attack and I told him yes I am. Then he went back to the same usual lovey-dovey topics.”
Jurors heard how the couple shared their enthusiasm for TV gameshows such as Deal or No Deal, but also fantasised about killing US President Barack Obama and exchanged extremist material.
They discussed plans for attacks in the UK several times. In messages after her birthday in March, Hussain mentioned an attack for a third time.
He talked about “Tokarev” and “pineapples” – meaning guns and grenades – in relation to a proposed attack on the British Museum, the court heard.
After Hussain was killed in Syria, Boular told undercover MI5 officers she planned to carry out his plans for an attack in the UK and join him in martyrdom.
Rizlaine was shot when armed police moved in to arrest the gang on April 27 last year but made a full recovery. She was jailed for life with a minimum term of 16 years, having admitted preparing acts of terrorism.
Dich, from Vauxhall, south London, was jailed for six years and nine months, with an additional five years on licence, for helping Rizlaine. Barghouthi, who pleaded guilty to failing to alert authorities, was jailed for two years and four months.