The 16-year-old girl was arrested late Wednesday at an uncle's house and the 17-year-old boyfriend was taken into custody at his home in Pico Rivera, a community east of Los Angeles. Neither teen's name was released.
"Forensic evidence has linked these two juveniles," Los Angeles County Sheriff Lee Baca said Thursday.
Richard Flores, 42, his 17-year-old son, Richard Jr., 10-year-old son, Matthew, and 13-year-old daughter, Sylvia, were killed early Friday. Flores' 39-year-old-wife, also named Sylvia, was wounded and remains hospitalized.
Authorities had said previously that the attacks were carried out by an assailant who slipped into the Flores home as the family slept. A bloody knife was found outside the home's back door.
The filing of formal charges was pending. Officials would not specify a motive or say what the daughter's role in the slayings was. Baca called it one of the worst crimes in the county's history.
"Needless to say these murders defy logic, they defy humanity, they defy explanation," the sheriff said.
The case is just the most recent in a string of brutal murders allegedly committed by minors in the Los Angeles area within the last week.
CBS News Correspondent Vince Gonzalez says in Glendale, two teens were arrested for the torture and murder of two boys, aged 13 and 14. Their bodies were found on a school playground. Even L-A's District Attorney seemed stunned by the week's body count, describing it as "beyond comprehension".
In nearby San Bernardino County, investigators say a15-year-old told them she beat an elderly woman to death on a whim as her 13-year-old friend watched. There are concerns the murders could be the beginning of a grim, new trend.
But Professor Laura Gomez of the UCLA Law School is not so sure. "I want to caution against that kind of reading of what has been happening for the last few days and remind us that these are, after all, isolated cases."
But researchers say they are definitely seeing a trend toward less mercy and more punishment for underage killers.
This may provide some small comfort for law enforcement, which is worried about the fact that California's juvenile population will increase by one-third over the next 15 years.
In an effort to get tough now, prosecutors say they plan to take advantage of a new California law and try the young Los Angeles area murder suspects as adults. That could mean life in adult prisons if any of the teenagers are convicted of murder.