Darnell said she had just gotten home Monday night when she got the call.
Her mother, 43-year-old Missy Darnell, and Rachel Darnell's 10-year-old sister, Genevieve, had been struck and killed by a car while walking to a local convenience store.
Police said the 22-year-old driver was reaching for a bottle of water in the back seat when she accidently struck the victims.
"I don't think she's a bad person," said Olivia Darnell. "She made a mistake."
"I forgive her," Rachel Darnell added.
The loss has come too soon for the children. Their father, 47-year-old Kirk Darnell, died suddenly of a heart attack exactly eight months ago.
"They were all just coming around and starting to smile," said a friend of the family, Tina Stone. "Now this happens."
It's unclear who the children will live with.
A donation bucket circulated through the crowd at the vigil. The community is determined to get the family some help.
A fund has been created for donations to the family:
Darnell Memorial Fund
Antelope Valley Bank
31924 Crown Valley Road
Acton, CA 93510