The deaths were found by post-mortems to be from heatstroke, with temperatures being in the 90s (over 32C) and a medical examiner determined that they were homicides.
The children were two-year-old Juliet Ramirez and one-year-old Cavanaugh Ramirez, of Weatherford.
Their mother, Cynthia Randolph, originally said they locked themselves in her car, said officials.
Randoph said later that Juliet refused to get out of the car so she locked both of the young children inside to teach them a lesson, thinking they would get out on their own, according to an arrest affidavit.
Randolph was arrested in June on injury to a child counts and remains in jail, Parker County records showed.
Every year, around 37 children die in the US after being left in a car, according to Kids and Cars. The safety organisation said that the month of July was a record high for hot car deaths. This year, 29 children died of heatstroke in the US after being left in a car.
This represented "the highest we’ve ever had,” said Jan Null a meteorologist with the Department of Meteorology & Climate Science at San Jose State University. He has logged the statistics on the No Heatstroke website.The previous record as of 31 July was 28 deaths in 2010. Null explained it was “a year where we ended up with 49 deaths for the year.”
Figures were taken from media reports, finding that of the 700 child vehicular heatstroke deaths for a 19-year period (1998 through October 2016), 54 per cent were because a child had been "forgotten" by a caregiver.