Mario Toliver Jr., 14, was last seen Thursday afternoon in their grandmother's apartment in Oakland's Chinatown neighborhood.
Investigators believe he shot his 17-year-old sister, Justice Toliver.
The siblings' cousin, Marianna Gaston, 22, told affiliate KGO-TV she was told the pair had been fighting because Justice had reportedly bleached some of Mario's clothes.
Family members are asking the teenager to turn himself in.
"We love you; we're hurting just like you are hurting, please do the right thing," the siblings' grandmother, Wanda Brown, said.
Police say Mario is armed and dangerous. He was described as having dark hair, brown eyes, standing around five feet, five inches tall and weighing about 140 pounds.
A $10,000 reward is being offered for information leading to his arrest.
Anyone with information regarding his whereabouts was urged to contact the Oakland Police Department at (510) 238-3821.
KGO-TV and ABC News contributed to this report.