Police said there was no sign of forced entry at the home on the 1200 block of Tamarind Avenue. Police believe they were murdered at 7 or 8 a.m.
The family's white Infiniti was missing, but officials found the car Thursday night a few blocks away. Police said the deaths do not appear to be a murder-suicide.
Neighbors said the mother and father spoke very little English, but they knew the 8-year-old girl very well because she played with the other children in the neighborhood.
"She would come over and call for my youngest daughter to borrow the bike or maybe play a little bit outside here, right outside where we live," said neighbor Manny Abarca. "I'm just in complete shock."
Police are interviewing the 15-year-old, mother and neighbors, hoping to find information that will lead them to the killer.
Anyone with information is urged to call (877) LAPD-24-7.
Eyewitness News Reporters Melissa MacBride and Leslie Miller contributed to this report.
- Get more local news headlines from Los Angeles
- Have a news tip? Send your tips, video, or pictures