A teenager was found guilty of Monday of planning an attack on London’s British Museum as part of an ISIS plot, police and prosecutors said. Safaa Boular, 18, plotted the grenade and gun attack in her home town after the authorities intervened to stop her travelling to Syria to marry an ISIS fighter she had met online. Her mother and sister admitted helping her, making the case Britains first involving an all-female cell of ISIS group-inspired plotters. Boular is the youngest female to be charged with planning an ISIS attack in Britain. She was only 16 when she made contact with British-born ISIS fighter Naweed Hussain, 32, discussing marriage and how they would don his-and-hers suicide belts. But her hopes of joining him were dashed when she was stopped at the airport in August 2016 following a family trip to Morocco, and her passport was confiscated. Instead Boular decided to plan an attack in Britain, detailing it in coded language -- grenades were "pineapples" -- to online contacts who were in fact undercover agents. Hussain was later killed in a drone strike. When Boular was charged with preparing terrorist acts in April 2017, she passed the baton to her sister Rizlaine, 22, and their mother Mina Dich, 44, who hatched their own plan. The trio were taped talking about an Alice in Wonderland-themed tea party, which the prosecution argued was code for an attack. Rizlaine and her mother were arrested after being tracked by police visiting potential sites around Westminster and buying knives. They pleaded guilty to terror offences, along with a fourth woman, Rizlaines friend Khawla Barghouthi, 21, who later admitted failing to alert the authorities. Safaa Boular denied the plot but was found guilty on Monday after a trial. Dean Haydon, Britains top counter-terror police officer, said the plot "involved a family with murderous intent, the first all-female terrorist plot in the UK connected to ISIS.” He added: "All three women were filled with hate and toxic ideology and were determined to carry out a terrorist attack. "Had they been successful, it could well have resulted in people being killed or seriously injured." "As a family unit, they are pretty dysfunctional," Haydon added. Rizlaine, her mother and their friend will be sentenced on June 15, although a date has not yet been set for Safaa Boulars sentencing. The British Museum houses important artifacts and art works acquired during the heyday of the British Empire, including the Rosetta Stone and sculptures from the Parthenon in Athens.