HAYWARD, Calif. (AP) _ A teen-ager who was arrested after she wrote ''Dear Diary ... I killed my little sister'' was convicted Friday of murdering her 4-year-old sibling.

A juvenile court judge convicted the girl of second-degree murder, saying the 4-year-old died of suffocation at the hands of her older sister.

The 15-year-old girl's mother collapsed after the ruling by Judge Sandra Margulies and was taken to a hospital.

Because the teen is a minor, she was not identified and could not be sentenced to incarceration beyond her 25th birthday. Defense attorneys, who said an appeal was likely, believe the judge may sentence her to a psychiatric hospital.

Sentencing was set for July 2.

Defense Attorney Shelley Antonio said the teen-ager was ''very hurt'' and could not believe a ''crazy diary entry'' had led to her conviction.

The county coroner testified that an autopsy showed the 4-year-old had been abused. Prosecutor Matt Golde said in closing arguments that he believed the teen-ager had killed her sister in a misguided effort to spare her from abuse.

An expert witness for the defense testified that there had been no sexual abuse, but the defense characterized the home as dysfunctional.

The 4-year-old's father, who is the teen's stepfather, denied allegations that he sexually abused his daughters.

Golde has not commented on why charges have not been brought in the alleged sexual abuse. He said Friday ''my door is open,'' but said the teen-ager would have to press charges.

The child died in August of unexplained causes. The teen made the diary entry Dec. 26, after getting the book as a Christmas present.

She was arrested in December after running away and her parents discovered the diary entry.

Antonio insisted the teen loved her sister too much to hurt her.

''Their relationship was an oasis in the center of a storm, in a hurricane of pain,'' she said. Antonio said the younger child died of asthma.