At about 5:20 p.m., 74-year-old Sharon Hufford and her 53-year-old daughter-in-law Susan Hufford were sitting in the back seat of a Toyota Camry that was heading east on Highway 12 when the car slowed in traffic and was struck from behind by a pickup truck.
CHP Officer Guillermo Garcia says the impact obliterated the back seat of the Camry and pushed the car forward into a third vehicle.
Officials say both died instantly.
According to the CHP, two men sitting in the front seats of the Camry suffered moderate and major injuries, and two people in the third vehicle involved were not injured.
The driver of the truck, who was later identified as 30-year-old Rohnert Park resident Nicholas Lee Tognozzi, was not injured.
Susan worked as a music teacher for the Mark West Union School District. The district superintendent described her in an email to parents as a "magnificent teacher who touched so many lives."
The CHP says Tognozzi told officers he looked down to read a text message right before the crash.
He was arrested for vehicular manslaughter and suspected drunk driving and booked at Sonoma County Jail.
The emergency response to the crash, which occurred just west of Farmers Lane, shut down the roadway for nearly three hours.
(ABC7 News contributed to this report)