Investigators are not ruling out that the driver may have fallen asleep behind the wheel.
The Tandel family from San Bruno was shopping for gifts in Gilroy to take with them to India for the eldest daughter's wedding.
As they returned home just before sunrise, the family's SUV drifted off the road and hit the back of a CHP car on Highway 101 near Palo Alto's Oregon exit.
The CHP officer who had pulled off the highway to help a driver change a flat tire suffered minor injuries.
The sisters who died from their injuries are identified as 24-year-old Nisha Tandel and 20-year-old Sheetal Tandel. Those injured include third sister 22-year-old Payal, an underage sister whose name was not released, their father Arvind, and his wife Yogita.