HAYWARD, Calif. (AP) _ Her lawyer insisted a 15-year-old girl loved her little sister too much to ever harm her. But the teen-ager had also written ''Dear Diary ... I killed my little sister.''

A juvenile court judge convicted the teen-ager of second-degree murder on Friday after a non-jury trial, saying the 4-year-old died of suffocation at the hands of her older sister.

The prosecutor said in closing arguments the killing was probably a misguided attempt to end the sexual abuse an autopsy showed the 4-year-old had suffered.

But defense attorney Shelley Antonio argued the little girl died of asthma and the diary entry was a troubled teen's confused attempt to give her mother some answer to the mysterious death.

''Their relationship was an oasis in the center of a storm, in a hurricane of pain,'' Antonio said. ''If there isn't reasonable doubt in this case, then it doesn't exist.''

The teen-ager was arrested in December after her parents searched her room when she ran away from their Fremont home in the east San Francisco Bay area.

They found the diary entry, made about four months after the 4-year-old had died, and eventually turned it over to police.

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

Sentencing was set for July 2.

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

While the teen-ager sat resolute and still as the findings were announced, her mother wept.

The distraught woman later fainted outside the courtroom and was taken to a hospital for treatment.

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.

A pathologist testified that an autopsy showed the 4-year-old had been abused at or near the time of death and at least once in the past.

Prosecutor Matt Golde previously has declined to comment why charges have not been brought in the alleged sexual abuse. He said Friday that the teen- ager would have to come forward herself.

Antonio argued that the teen's diary description of putting her hand over the 4-year-old's mouth did not fit with a pathologist's finding that someone crushed the girl's chest or back.

Still, Golde said the diary entry was real.

''People do not just confess to murder,'' he said.