Safaa Boular, now 18, secretly discussed the murderous plans with her sister and mother, using coded language which had an Alice in Wonderland tea party theme.
But instead of cucumber sandwiches and cakes, her sister Rizlaine, 22, bought a large knife to Westminster in April last year.
Boular first began planning a grenade and gun attack on the British Museum when she was thwarted from joining her husband, who was a member of ISIS in Syria.
She passed the baton to her sibling after she was arrested and remanded in custody for trying to travel to the war zone.
Following an Old Bailey trial, she was found guilty of preparing acts of terrorism abroad and in the UK.
Boular made no reaction in the dock as she was found guilty on Monday by a jury after two days of deliberations.
Judge Mark Dennis QC put off sentencing for around six weeks for a report to be compiled.
Boular is the youngest female to be charged with planning an ISIS attack in the UK. Her fellow plotters admitted their roles before the trial and they will be sentenced at a later date.
The women were snared in a “pro-active” investigation involving surveillance by counter-terrorism police and MI5 agents posing online as IS operatives.
Counter-terrorism chief Dean Haydon, of Scotland Yard, said the case demonstrated a worrying rise in youngsters being arrested for terrorism.
The court heard how Boular was just 16 when she was wooed online by Coventry-born Isis fighter Naweed Hussain, 32.
The couple got married in an online ceremony and talked of donning his-and-hers suicide belts to achieve martyrdom together.
Police uncovered Boular’s plans to join him following an airport stop in August 2016 and confiscated her passport.
While on bail, Boular turned her attention to an attack on the British Museum, encouraged by Hussain in “lovey-dovey” messages.
Boular also scoped out the MI6 headquarters near her home, and took a selfie in front of the building, the court heard.
Hussain was lured into revealing his intentions to British secret service agents posing as ISIS supporters online, before he was killed in a drone strike.
When an agent pretending to be his commander informed Boular of his death on April 4 last year, she was wracked by grief and resolved to join him.
She revealed to the undercover officer that Hussain had talked about attacking the British Museum with a “tokarev” Russian-made pistol and “pineapples” – code for grenades.
On being remanded in custody over her attempt to travel to Syria, Boular persuaded fellow Isis supporter Rizlaine Boular to take up the baton.
In coded telephone calls involving their mother Mina Dich, 44, the sisters discussed a traditional English tea party with an Alice in Wonderland theme.
Rizlaine Boular said she knew “a few recipes for some amazing cakes” for a “proper like English tea party kind of thing”.
Safaa Boular suggested a “Alice in Wonderland theme” telling her sister: “You can be the Mad Hatter ’cause your hair’s crazy.”
Mother-of-four Dich responded: “That will be fun.”
Rizlaine Boular then set about reconnaissance around the Palace of Westminster, and bought knives and a rucksack from Sainsbury’s.
She was accompanied by Dich, unaware that they were under surveillance by counter-terrorism police.
Rizlaine Boular shared her plans with her friend Khawla Barghouthi, 21, and even practised a knife attack at her home in Willesden, north west London.
She was shot when armed police moved in to arrest the gang but went on to make a full recovery.
Safaa Boular, who lived at home in the “dysfunctional” family, claimed she had been groomed by Hussain, who she met online through a female Isis recruiter in the wake of the Paris terror attacks.
She wanted to get married and live peacefully in Raqqa, where women were not called “umbrella” or “post box” for wearing Islamic dress, she said.
Three times, she said Hussain urged her to attack the UK – over Christmas 2016, Valentine’s Day and around her birthday in March last year – but she rebuffed him.
Boular, who viewed gory beheadings and chatted about killing US President Barack Obama, told jurors: “Nothing online is real.”
In the year since she had been in custody, she said she had changed and now opts to wear western clothes rather than a burka.
Rizlaine Boular, and Dich, from Vauxhall, south London, pleaded guilty to preparing acts of terrorism and Barghouthi admitted failing to alert authorities.
Haydon, Senior National Coordinator for Counter Terrorism, said: “This was without doubt a major investigation, a proactive investigation. This involved a family with murderous intent, the first all-female terrorist plot in the UK connected to Daesh.
“It’s difficult to say if we will see more females. We have seen young children involved in martyrdom attacks. We have seen Daesh using young children.
“Here in the UK, we have seen an increase in young people, teenagers, getting involved.
“Arrests of youngsters have increased over the last 12 months and clearly that is a concern for us.”
He described Safaa Boular as a “confident, articulate, intelligent and a relatively mature 18-year-old” who had been “quite devious” in the way she hid her plans
He added: “As a family unit they are pretty dysfunctional. On the evidence we can see they had access to a vast amount of extremist material.”