Teen-Ager Convicted of Murdering Her Northridge Neighbor : Crime: The girl is one of three sisters accused in the 1991 slaying. Her siblings will be tried later.

One of three teen-age sisters charged with fatally stabbing a 62-year-old Northridge neighbor and then bragging about it to friends has been convicted of murder in Juvenile Court, authorities said Thursday.

The middle sister, who was 15 at the time of the November, 1991, slaying, stabbed Meta Frances Murphy at least 11 times in Murphy’s condominium, while her older sister allegedly turned up the stereo to drown out the screams.

“I’ve never handled a case like this before--it’s just stunning,” Deputy Dist. Atty. Thomas Rubinson said. “Such young girls, an elderly victim and the brutality of it. It’s shocking.”

Murphy, a librarian who lived alone in a Mayall Street condominium complex, opened her unit to the girls whenever they wanted snacks or a cold drink.

She was killed shortly after returning from the store with groceries, and her body was found in an upstairs closet, covered with clothes.

Sylmar Juvenile Court Commissioner Jack Gold on Wednesday sustained the petition charging the middle sister with murder, a Juvenile Court procedure equivalent to conviction in a higher court. The conviction also contained the special allegation of using a knife.

The girl faces a maximum of nine years in a California Youth Authority correctional facility at a Jan. 28 sentencing hearing.

Her sisters, now ages 13 and 17, face murder trials in Juvenile Court in February and March. They are accused of helping plot the killing.

All three sisters have remained unidentified because they are under the age of 18.

The slaying, which rocked the quiet residential complex, went unsolved for months until at least one acquaintance of the sisters told police that she heard the two older girls boasting about the slaying, authorities said.

“What’s the motive? I still don’t know,” Rubinson said.

Eleven witnesses were called during the trial, including two youths who said the defendant bragged about killing Murphy.