Justice Toliver, 17, was fatally shot Thursday afternoon in her family's apartment in Oakland's Chinatown neighborhood, KGO-TV, San Francisco, reported.
Police were searching for her brother, 14-year-old Mario Toliver Jr., who is suspected of killing his sister.
Police described the boy as being armed and dangerous.
"We have not recovered the firearm that was used in this homicide so that is a concern not only for law enforcement but for our community as well," Oakland police spokeswoman Johnna Watson said.
Surveillance cameras in the apartment building caught Mario Toliver leaving through an emergency exit with the gun, police said.
Family members said the brother and sister had gotten into an argument over Justice bleaching Mario's clothes.