Olivia Culbreath, 21, of Fontana, was listed in stable condition, said Rosa Saca, spokeswoman at Los Angeles County-USC Medical Center.
Culbreath received a broken femur and a ruptured bladder in the early Sunday morning crash on State Route 60 in Diamond Bar, a bedroom community 35 miles east of downtown Los Angeles.
She was arrested on suspicion of drunken driving and manslaughter after being pulled from the mangled Camaro, California Highway Patrol Officer Rodrigo Jimenez said.
An alcoholic beverage was found at the scene, authorities said.
Witnesses said the Camaro may have been traveling at up to 100 mph eastbound in westbound lanes when it collided head-on with a Ford Explorer, hurling wreckage and bodies onto the freeway.
A Ford Freestyle then hit the Explorer. The driver, Joel Cortez, 57, of La Puente, was treated for minor injuries.
Two people in the Explorer—Gregorio Mejia-Martinez, 47, and Ester Delgado—died at the scene, county coroner's Lt. Fred Corral said. Two other family members in the SUV—Leticia Ibarra, 42, and Jessica Jasmine Mejia, 20—were declared dead at a hospital. All four were from Huntington Park. Delgado's age was not available.
Culbreath's sister, Maya Louise Culbreath, 24, of Rialto, and another Camaro passenger, Kristin Melissa Young, 24, of Chino, died at the scene, Corral said.
The family was still trying to cope with the tragedy Monday, said Carole Phillips, Culbreath's grandmother.
"They won't let us see her," Phillips said by telephone. "We're just devastated because of the loss of the others, too. It's just too much."
Wendell Johnson of Rancho Cucamonga, a friend, told the San Gabriel Valley Tribune ( http://bit.ly/1egcAhY) that the group in the Camaro had been on a "girls' night out" in Fullerton.
Neighbors in Fontana told the paper that Culbreath had given birth to a son in late January.
Culbreath had a 2010 conviction for driving under the influence of alcohol stemming from a 2009 incident when she was 17. Her license was pulled, but it was reinstated in 2011, according to Department of Motor Vehicle records.
However, Octavio Castellanos, a neighbor of Culbreath, told the Tribune said he rarely saw her drinking before she became pregnant and added that she didn't drink at all during the pregnancy.
"You wouldn't think she would do something like that," said his brother, Andretti Castellanos. "She was the type of girl that never got into trouble."