Florida - A 6-year-old boy beat his newborn sister to death after their 62-year-old mother left her young children alone in a car for more than a half hour while she went to get her cellphone fixed, authorities on Florida's Gulf Coast said.Now the mother, Kathleen Marie Steele, is charged with aggravated manslaughter of a child.Pinellas County Sheriff Bob Gulatieri said she went to a cellphone repair business in St Petersburg on Monday and left her children in the car - the infant, a 3-year-old and a 6-year-old.While Steele was gone, the baby began crying and the 6-year-old began beating her to make her stop, the sheriff said."He was tossing that baby around like a rag doll," the sheriff said during a press conference on Thursday, shortly after Steele was arrested.Nevertheless, "The mistake is with the adult, Kathleen Steele, not a 6-year-old boy," the sheriff said.Gulatieri said the boy won't face criminal charges. His mother, who lives in the St Petersburg area, made her first court appearance on Friday, saying only that she understood the charges. Her two sons have been placed in therapeutic foster care. Her attorney Robert Love said he wouldn't contest the state's order for Steele to have no contact with children 18 or younger.Gulatieri painted a picture of a single mother who seemed overwhelmed and ill equipped to care for her three young children in the months leading up to the baby's death. He said Kathleen Steele's husband died of cancer in 2011 and she paid a significant sum to be artificially inseminated with his frozen sperm to conceive her 3-year-old son and newborn daughter, who was also named Kathleen.She told authorities she even hoped to have more children, Gulatieri said, even though friends and family said she was struggling to raise the children on her own. The two boys were "running amok and were unsupervised" to the point that an anonymous caller contacted the state's child protective hotline about them on August 2.Steele — who once lived in a half-million-dollar home — filed for bankruptcy last year, and an accidental fire forced the family to stay in a hotel last month. While there, the fire alarm went off and Steele told deputies she put little Kathleen in the car carrier but tripped going down the stairs and the baby tumbled out, striking her head. The baby, diagnosed with a minor brain bleed, stayed for two nights in the hospital, Gulatieri said. It was reported to the child abuse hotline, but the sheriff's office said they did a thorough investigation and it appeared to be an accident.Days later, a seemingly frazzled Steele took the three children to a pediatrician amid concerns the 13-day-old baby wasn't eating. The doctor found nothing wrong and the family then headed to get Steele's cellphone fixed after the 6-year-old dropped it. The sheriff said video shows Steele left the children alone for 38 minutes while she was in the repair store.The 6-year-old told investigators the baby started fussing and he tried to calm her. But deputies said the boy later used a doll to show how he repeatedly slammed the infant's head into the minivan's ceiling, dropping her on the floor, flipping her over and pummeling her.The van's ceiling was covered in blood and investigators said the baby was likely dead when her mother returned to the car. Her elder son tried to tell her something was wrong with the baby, but authorities said she disregarded him and stopped at a rental car company before heading home. That's when she noticed the baby's injuries."The baby was beaten and traumatised," the sheriff added. "There was gross swelling in her face. Her skull was cracked in numerous places."Even so, Gulatieri said Steele didn't call 911. Instead, she called a neighbor who is a nurse. The nurse recognised the baby was dead, but performed CPR until paramedics arrived.Gulatieri called the 6-year-old bright and mature for his age, and said he even reminded his mother that they needed to renew their rental car lease."The things that he said and his awareness level and how astute that he is, it's amazing really. By hearing him talk you'd never think the kid was 6," Gulatieri said.But the boy also had aggression issues, he said. One neighbor told authorities she wouldn't let her son play with him. Another adult said the boy sucker punched him."By numerous witness accounts, Kathleen Steele was an inattentive parent and (the boys) were largely unsupervised and had very serious behavior issues," the sheriff said.Elementary school officials said the boy will not return to the school.When asked whether the boy was aware he'd killed his sister, the sheriff said he made a vague reference to his actions."At one point he said, sometimes people make really bad mistakes."Steele's relatives were apparently upset with her decision to continue getting pregnant and raising young children by herself."It caused some dissension within her family that she was doing this because they realized this was something that she should not be doing and it was wrong," said Gulatieri, who said she was inseminated by a doctor in New York.Steele and her husband had appeared on an episode of the reality TV episode titled "I'm Pregnant and 55 Years Old."The sheriff's child protective investigators had been scheduled to evaluate Steele on Aug. 1 about the fall that landed the baby in the hospital. However, she canceled and had rescheduled for Aug 10 — two days after little Kathleen died.When asked about the anonymous call to the child abuse hotline last week, the sheriff said investigators went to the home and met with the family, even though the concerns were vague."I'm very confident in this case that our child protection division did everything right ... there was no way that anybody in hindsight would look at this and say we should have done something differently."