A university student who stabbed her younger sister to death will spend at least four years and three months in jail.

Kathleen Worrall, 22, was charged with murdering her 18-year-old sister Susan in their Strathfield home in October 2008.

The Crown accepted her guilty plea to a lesser charge of manslaughter due to mental illness. The court heard she had a rare hormonal condition - congenital adrenal hyperplasia - and at the time of the attack had not been taking her medication.

Susan Worrall, a Year 12 student at the Methodist Ladies College in Burwood, suffered 50 stab wounds to her back and neck. She was stabbed after getting out of the bath.

After stabbing her younger sister, Kathleen Worrall phoned her father, saying "Daddy, I've stabbed Susan. I've got blood all over me."

The New South Wales Supreme Court heard a week before the attack the sisters fought over a hair straightener and Kathleen Worrall told a friend, "I've had enough."

During sentencing submissions earlier this year, the court heard Worrall once told a psychiatrist her sister was "Miss Cute and Popular" but people who knew her knew that was not true.

But her mother said her daughter is sorry for what she has done and would "take it all back in a moment' if she could.

In handing down a sentence today, Justice Elizabeth Fullerton said Kathleen Worrall has loving parents and is unlikely to re-offend.

The judge said the incident was a "compound tragedy" for the girls' parents.

She said the "horrific death" would not have occurred without Kathleen's medical condition.

Worrall cried in court and looked across at her mother and father.

She whispered "I love you" to them before she was led away. Outside they made no comment.

With time already served she will be eligible for parole in 2013.