Asma Begum said her “world fell apart” when Ms Begum left aged just 15 to travel from Bethnal Green, East London, through Turkey and into ISIS-controlled territory.
She said she has not touched anything in her bedroom since she left, and her school blazer still hangs on the door of the front room, “just as it was when she left”.
The UK revoked Ms Begum’s citizenship in 2019, shortly before she was found living in a Syrian refugee camp, nine months pregnant.
In extracts of a statement shared on the third day of an appeal against the Home Office decision, Ms Begum’s mother said: “My youngest daughter is even more present in my mind, the one I think about almost every hour of every day.
“When she left home in 2015, our worlds fell apart.”
She continued: “Shamima and I shared a bedroom and I have not moved anything of hers from our room.
“Her drawers are still full, her perfume, pens and jewellery, her clothes are still there. Her pyjamas are folded neatly.
“The box with her school books I look at sometimes.
“Her school blazer is still hanging on the door in the front room, just as it was when she left.”
Mrs Begum added: “On the last birthday she spent with us before she left, she did not want a cake but wanted a pizza with candles on it instead.
“She was so happy that day. On each of her birthdays since she left we order pizza and still celebrate her birthdays.
“It was always sad but we look forward to the birthday party we will have when she is back with us.”
Ms Begum's barrister, Dan Squires KC, who read the statement, said it was “a powerful indication of the impact and the connection that remains between the appellant [Ms Begum] and her family here”.
The barrister said the decision to deprive Ms Begum of her citizenship “extinguished her family's ability to rebuild their family life with their daughter and sibling with whom they lost contact as a result of the acts of criminals in targeting a young child, and trafficking her out of the country”.
He added that the family were not considered in the decision to remove Ms Begum's citizenship, breaching their human right to a family life.
Lawyers for the Home Office, who are due to make their oral arguments on Thursday, said that Ms Begum was in Syria “as a result of her own choice to leave the UK”.
Sir James Eadie KC, for the department, continued in written submissions: “It is not appropriate to start from the point at which Ms Begum left the UK, and assume family ties would have remained the same.
“Generally speaking, a prolonged period of separation weakens family ties, rather than bolstering them.”
Ms Begum's lawyers also argued that former home secretary Sajid Javid failed to properly consider whether the use of the power to deprive someone of their citizenship was disproportionately applied to British Muslims or if it “impacts detrimentally upon the relations between members of Muslim or particular minority ethnic communities and others.”
Earlier evidence produced as part of the appeal at the request of Ms Begum’s lawyers heard from a former MI6 director of counter-terrorism, who said the UK government was “fundamentally misguided” in its approach to her case.
Former co-ordinator of the UN Al Qaeda and Taliban monitoring team Richard Barrett said that depriving Ms Begum of her British citizenship in 2019 — when she was 19 years old — “undermines” the UK’s role in international counter-terrorism.
A written opinion by Mr Barrett and Paul Jordan — head of responding to violent extremism at the European Institute of Peace — was produced as part of Ms Begum’s appeal against the Home Office’s decision at the request of her lawyers.
The two experts said there was likely to be a higher national security risk caused by refusing to bring people back from Syrian camps.
The hearing in London before Mr Justice Jay is due to finish on Friday, with a decision expected in writing at a later date.