Officials say the victim, Justice Toliver, was shot by her brother, Mario Toliver, at the apartment complex on Franklin Street just after noon on Thursday.
The victim's cousin, 22-year-old Oakland resident Marianna Gaston, says she had been told the boy was mad at his sister for bleaching his clothes.
When she first heard about the shooting, Gaston said she thought it was "a joke" and asked, "Why would he kill her over that?"
Standing outside the 12-story apartment building several hours after the shooting, Gaston said, "Nothing makes sense. It doesn't add up."
Police note that they have not yet determined the motive for the shooting. They are still searching for Mario.
We spoke with Justice and Mario's grandmother, who 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 Mario."
"And I've been there for them ever since they came out of the womb. You know, I never went straight on my kids. I don't understand, it's just the streets. The devil is working all the time, devil's working all the time," said Mario Toliver, Justice and Mario's father.
Justice was the mother of 3-year-old girl. Mario is described as five feet, slim, and wearing a black hooded sweatshirt.
Oakland police said their investigation into the shooting is ongoing and anyone with information about it should call their homicide section at (510) 238-3821.
(Bay City News contributed to this report)