Begum was 15 when she fled east London with two other friends to travel to Syria to marry IS fighters in 2015 at a time when the group's online recruitment program lured many impressionable young people to its self-proclaimed caliphate.
Begum, now 19, resurfaced at a refugee camp in Syria and recently told reporters she wanted to come home. But her apparent lack of remorse has triggered criticism in Britain and the family has expressed its own shock at her lack of repentance.
"We are sickened by the comments she has made, but, as a family man yourself, we hope you will understand that we, as her family, cannot simply abandon her," her sister, Renu Begum, wrote in the letter to Javid.
"We hope you understand our position in this respect and why we must, therefore, assist Shamima in challenging your decision to take away the one thing that is her only hope at rehabilitation, her British citizenship," she added.
The situation with Shamima Begum's citizenship is complex and will likely be argued in the courts. Javid has revoked her citizenship — even while he said he would not make a decision that would render a person stateless. Her family has insisted she is not a dual citizen.
Although it's unclear if Begum has committed a crime, her comments throw into sharp relief larger questions about how Western societies will deal with others who joined IS, but want to return to their home countries now that the extremist group is on the verge of collapse.
Her family stressed they rejected Begum's comments to British media in which she said she did not regret travelling to Syria to join extremist fighters. "That year we lost Shamima to a murderous and misogynistic cult," the letter read. "My sister has been in their thrall now for four years, and it is clear to me that her exploitation at their hands has fundamentally damaged her."
Javid has suggested the dispute about her citizenship will not affect her child. "If a parent does lose their British citizenship, it does not affect the rights of their child," he told the House of Commons.
Other IS brides are also finding themselves in the courts. The father of an Alabama woman, 24-year-old Hoda Muthana, who also joined Islamic State, has filed suit against the Trump administration so she can be allowed to return to the United States with her toddler son.
Muthana's case is different than Begum's, however, because U.S. officials argue that she was never a citizen in the first place — a premise denied by her family.