Reline Harris, 11, and Derinne Harris, 12, were last seen Monday night in Lancaster.
Authorities said they may be with their biological father, Deshaun Carter, 30. He is described as an African-American male, about 5 feet 5 inches, weighing 220 pounds with black hair and brown eyes. His last known address is in Oregon.
Anyone who sees them or has information is asked to call the Lancaster Sheriff's Station at (661)948-8466.