The victim, identified as Justice Toliver by her family, was shot and killed Thursday afternoon in a Chinatown apartment, allegedly by her brother, Mario Toliver Jr., the Oakland Tribune reported.
Mario Toliver fled the scene after the shooting.
"I love him. No love is lost," said Wanda Brown, the children's grandmother. "I know he's hurt right now, and I know they were bickering, but they were like two peas in a pod."
"If you know him, if you see him, we just want him to come home. We know he's probably scared. We love him," cousin Gregory Stewart, 25, added.
Brown said the two grew up together in the apartment where the shooting occurred. Their father, Mario Toliver Sr., was not home at the time of the incident.
Police have not commented on the possible motive, but Stewart said the two had gotten into an argument recently because the girl had bleached some of the boy's clothes.
"They got into some type of altercation," Stewart said. "Whatever it was about it shouldn't get that serious to pick up a gun and kill his sister."