Oakland police describe 14-year-old Mario Toliver as 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 spokesperson Johnna Watson said.
Mario is five feet, five inches tall, 140 pounds with dark hair, brown eyes.
Police say Mario shot his sister, 17-year-old Justice Toliver, inside the apartment they shared with their grandmother on Franklin Street in Oakland's Chinatown neighborhood.
The surveillance cameras inside the apartment building caught Mario walking down the hallway on the fifth floor with a gun. He then ran out an emergency exit that leads to Franklin Street.
A cousin told ABC7 News the two siblings had argued over how she was bleaching his clothes.
"It doesn't make sense to me; if that's the reason for him to cause that, that's crazy," property manager Thomas Chak said.
The victim worked at a McDonald's restaurant on Market Street in Oakland. Justice Toliver also had a 3-year-old daughter.
Thursday night, family members were trying to convince the 14-year-old to turn himself in.
"We love you; we're hurting just like you are hurting, please do the right thing," grandmother Wanda Brown said.
Police say the boy's family has been helping in the investigation.