Oakland police spokeswoman Johnna Watson said officers received a report of the shooting just after noon Thursday and found the girl in a downtown apartment with a fatal gunshot wound. The victim was pronounced dead by emergency medical workers.
Family members tell KTVU-TV that the girl was shot after getting into an argument with her brother over laundry. They say she had bleached some of his clothing.
Police have not confirmed a motive. The investigation is ongoing.
KGO-TV reports the victim, Justice Toliver, was allegedly shot by her brother at a 12-story apartment complex on Franklin Street.
Standing outside the apartment building several hours after the shooting, the victim's cousin, 22-year-old Oakland resident Marianna Gaston said, "Nothing makes sense. It doesn't add up."
The suspect's grandmother begged her grandson to come forward, "So once again, grandson, grandma is asking you, please do the right thing, please," she said. "And I love you, I love you with all my heart."
The Associated Press contributed to this report.