Grace Dare, now calling herself Khadijah Dare, was a devout Catholic until 2010 when she converted and became a Muslim.
The 25-year-old was said to have changed dramatically after turning to Islam and soon after fled the UK with her one year old son to join IS.
It's believed that the boy, now four years old and named Isa, is the toddler pictured in the latest IS propaganda video which shows the killing of five men.
Now her father, Sunday Dare, has spoken about the extremist's upbringing.
He told The Sun she was religious and "would read her Bible all the time".
"We called her Grace after the Amazing Grace hymn," he said.
"She went to church every week without fail, often three times. She was always smiling and singing at church, sat in the pews with all the grown-ups.
"She would go with me, mum Victoria and brother Nathaniel as a family. We would dress up in our Sunday best.
"If she read her Bible she would ask me about the stories. She wouldn't really read anything else.
"I thought it was unusual she wouldn't read anything else.
"After me she was the most religious in the family."
The boy, thought to be her son, makes a fleeting appearance in a desert landscape at the end of the 10-minute video.
Wearing camouflage clothing and a black headband, he speaks in what sounds like a British accent and says: "We are going to go kill the kaffir (non-believers) over there."
She travelled to Syria in 2012 and married Swedish Islamic fighter Abu Bakr, who is believed to have been killed since.