It's understood that the three-year-old girls snuck into the car when Foster wasn't paying attention where temperatures were estimated to have reached 52 degrees (130 farhenheit).
The girls were last seen by their caregiver who put them to bed at the home in Georgia, US.
After missing for several hours, their bodies were discovered in a car outside the home.
Foster is said to have immediately turned herself into police who laid charges on her including two counts of second degree murder and second degree cruelty to children.
The girl's biological mother has shared her devastation over the passing of her children, explaining that they had only lost custody in the first place as she was still grieving the premature death of another child.
'We have already lost one daughter in 2014 and six months after she passed we found out we were having girls,' she said.
'Losing a child is the worst possible thing you could ever go through,' she added.
A GoFundMe has been set up by a family member to raise money for funeral expenses.