The younger child's death was confirmed by a spokesperson for the California Highway Patrol's Newhall office. Her name was not immediately released.
Her sister, Kathleen Lopez, succumbed to her injuries on the day of the crash and was identified Wednesday by the Los Angeles County coroner's office.
According to investigators, the family's car was involved in an initial crash on The Old Road just south Pico Canyon Road shortly before 9:45 a.m. Monday The driver then ran a red light and slammed into a power pole at the intersection, the CHP said.
Several good Samaritans rushed to help, extricating the trapped girls seconds before the car erupted in flames.
The mother, who has not been publicly identified, was listed in stable condition.
The cause of the crash is under investigation, the Highway Patrol said. No citations or arrests were immediately announced.