A teenage mum has been jailed for 40 years after she left her two daughters to die in a 48 degree car while she smoked marijuana and partied with friends.

A judge has slammed Amanda Hawkins, a self-described sex-addict, for her selfish behaviour, telling her, "People in our community take better care of their pets than you took care of your kids" after she left her two little girls, Addyson Overgard-Eddy, 2 and Brynn Hawkins, 1, to die in a car for 15 hours.

The Texas woman left the children in the vehicle on June 6, 2017, around 9pm where they sat helpless, strapped in their car seats until noon the next day.

"Those precious little girls would still be here today if this had not happened," Kerr County, Texas, Judge N. Keith Williams told the now 20-year-old during her sentencing last week.

The toddlers were found in a serious condition and close to death as temperatures soared to nearly 50 degrees, news.com.au reported.

In September, she admitted to leaving her kids in the car to die, having pleaded guilty to two counts of reckless injury to a child and two counts of abandoning or endangering a child.

Judge Williams sentenced her to 40 years jail.

According to Hill Country Creaking News, he would have sentenced Hawkins to further punishment if it were possible.

It was discovered that Hawkins had left them in a vehicle all night after smoking marijuana with several people inside of a shed — even taking her youngest child into a shed where multiple people were smoking marijuana, before putting her back in the vehicle, the publication reported.

She didn't take the girls to hospital because she "didn't want to go to jail".

She ran cold water on them in a bathtub before taking them to a medical centre two hours later. They were transferred to University Hospital in San Antonio, where Dr John Gebhart tried to save their lives over the next 40 hours, according to The Sun.

He told the court the little girls most likely went into "profound" shock while they were left in the hot car.

Dr Gebhart told the judge they most likely had very painful deaths with brain swelling having been the cause of death.

A 16-year-old boy, Kevin Franke, who had slept in the car with the girls, and accompanied Hawkins to the hospital claimed he was unaware the girls were in the car while he was sleeping.

The now 18-year-old who was later arrested, pleaded not guilty in April to two counts of manslaughter, two counts of injury to a child and two counts of child endangerment.

He is due to appear in court in January next year.