The mother accused gross vehicular manslaughter following the death of two children also faces a felony hit-and-run from a crash last year.
The 22-year-old was driving a Kia Sorento eastbound on Otay Lakes Road around 6 p.m. when she lost control near Wueste Road, veered off the side of the road and into the water, according to the California Highway Patrol.
There were three passengers: Her daughter, a 31-year-old male passenger and his daughter. Hernandez and the male passenger managed to escape the car, but the two girls drowned as they were trapped under water.
Hernandez appeared in court Thursday for a status hearing, but instead she was formally charged with the two new felonies.
She will be back in court next month for a status hearing and her preliminary hearing is scheduled for October.
"Most defense attorneys would try to separate those two cases and try them separately," said Brian Watkins, a legal expert with Brian E. Watkins and Associates. "The downfall for her is, she's looking at two felony charges. She could get punished for two felony offenses and serve those sentences consecutive to one another."