The family entered Syria around five years ago, travelling to Aleppo, on to Raqqa and then down the Euphrates river valley, ending up in Baghouz.
Human rights watchdogs have heaped pressure on the international community, particularly Western countries, to repatriate their citizens from Syria.
Few have done so, and the UK has revoked the citizenship of high-profile militants and Isil supporters, including Shamima Begum.
The case for orphans is more complex, and both France and the Netherlands have permitted the return of child nationals whose parents took them to Syria to join the terror group, but died there.
In the hours after the Ain Issa camp was overrun, aid agency Save the Children made a plea with countries to repatriate their citizens, particularly children, before it was too late.
By Sunday afternoon, Amira, Hiba and Hamza were gone.
Three British Islamic State orphans have gone missing in northern Syria after the camp they were in was shelled by Turkish forces.
Ten-year-old Amira, her sister Hiba, eight, and brother Hamza, around the same age as Hiba, were discovered in the Kurdish-run Ain Issa camp by a BBC reporting team last week.
But they vanished on Sunday after hundreds of women linked the caliphate fled the camp in what Kurdish-led forces described as a mass escape facilitated by Turkish forces.
They had been living with 21 other orphans at the camp since they were evacuated from the wreckage of Baghouz, Isil's last redoubt, in March.
The three siblings, whose surname is unknown, emerged from Isil's last stand bearing physical and emotional scars but still able to speak some English.
Their parents, older brother, and two other sisters were killed in the battle. Their parents, who have not been identified, brought the family to Isil's so-called Capilphate in Syria five years ago.
In an interview with the BBC, Amira, who still has traces of a London accent and is able to write, painstakingly wrote out the place she once called home: "LaNDN uKeH" -- London, UK.
Describing her pre-war life, she said: “I go to the park, I go to grand-mum’s house, I go to the fun fair."