The family believe the attack was targeted at the 15-year-old; the boy had been having trouble with a group of people who had threatened him several times.
His mother said she called the police 15-17 times about the threats since March.
"Typically in these type of situations they are usually retaliatory, so we will be looking into that," Sgt. Greg Pinkins said when asked about a motive, but added: "At this time, it would only be speculation."
"She was my baby," Sierra's father Michael said, per the broadcaster. "One moment she was there and the next she was gone."
"My girl didn't do nothing to deserve anything. She has cerebral palsy, laying in the bed and you shoot her in the face," he added, per Click2Houston.
"Whoever came and shot my house up and murdered my daughter, I want them caught."