The 11-year-old was pronounced dead at the scene.
"The second victim, a 14-year-old girl, was airlifted to Childrens Hospital. We don't have anything further on the condition," said L.A. County Sheriff's Lt. Patrick Nelson.
"There was one additional person inside the home," said Nelson. "Fortunately she was able to escape injury."
Although detectives said they responded to a shooting call, a source close to the investigation told Eyewitness News both victims were stabbed.
Two hours later, detectives arrested a suspect a few blocks from the crime scene hiding at a house.
"Through initial witness statements, deputies set up a containment, they found who they believe is the suspect that was outstanding," said Nelson. "He was detained without incident."
Investigators said the suspect is a 16-year-old male who lived in the same house as the girls. He is believed to be a cousin.
A motive for the stabbings has not been determined. Some speculate that it was because of a family dispute.
"To see something like this happening right across the street from you, it's terrifying," said neighbor Curtis Hallenbeck.
Hallenbeck said he was awakened by the screams of a teenage girl who ran from a nearby house saying her relatives had been stabbed by their cousin.
"She was able to get out," Hallenbeck said. "She saw her cousins were being attacked so she ran out of the house immediately.
"Hallenbeck's wife called 911 while he went up to the house, and found the 14-year-old girl bleeding in the driveway.
"She was saying that her younger sister was in the house and that she was also very hurt," Hallenbeck said.
Detectives said some family members were so distraught, at least one of them tried to break through the police barricade.
"It's my understanding that was a distraught family member trying to get into the crime scene," said Nelson. "Understandably, it's a very tragic situation and our hearts go out to the family.
"They're my nieces," said an aunt at the scene. "They're my nieces, oh God, please, please Jesus. There's the dad. There's the dad, there's the dad, there's the dad." The aunt embraced the distraught father.
Neighbor Andrea Rymer said her kids played together with the victims often.
"We knew the two little girls," said Rymer. "They were really good little girls, really well-mannered, really polite. I mean, this is ridiculous. I can understand the dad being as upset as he is. Nothing happens here like this."