Safaa Boular, now 18, secretly discussed the murderous scheme with her sister and mother using coded language which had an Alice in Wonderland tea party theme.
Her sister Rizlaine, 22, bought a large knife to bring carnage 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 IS husband 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 yesterday 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 IS 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 caught in a “proactive” 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 IS 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.
Burka-wearing 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 murderous intentions to British secret service agents posing as IS supporters online before he was killed in a drone strike.
When an agent pretending to be his commander informed Boular of his death on 4 April 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 IS 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.