The girls' friends cannot comprehend the death of Susan, 18, and the charging of her big sister Kathleen with the murder.

"They were like normal sisters. They fought occasionally, just like my sister and I do. I don't know what happened that day," Susan's friend, who did not want to be named, said yesterday.

Kathleen, 20, "was just as nice as Susan. She used to help us out at school and homework and things."

Kathleen, a psychology, theology and history student at Australian Catholic University, is under close medical observation in a special care area of Silverwater jail. She is due to appear in Burwood Local Court tomorrow.

She was arrested at the Worralls' Strathfield home last Friday. Emergency services had been called to the house in Mosely Street about 11.30am after reports of a stabbing. They found the body of Susan, a year 12 student at Methodist Ladies College at Burwood. She had at least one knife wound to her neck.

The MLC Burwood community held a memorial for Susan on Saturday.

Susan's friend said the death broke her heart and left other school friends distraught.

"A piece of my heart is gone and I can only imagine how the rest of her family is coping with this," she said. "My heart goes out to them."

The girls' mother, Maureen Worrall, told Parramatta Bail Court Kathleen needed medication and should be closely monitored against self-harm. Kathleen is a member of two Facebook support groups for people who suffer from the hormonal disorder congenital adrenal hyperplasia.

One group has closed to new members citing privacy concerns and recent "unfortunate events in Australia". The other warned members the disorder might attract negative media attention because "a young woman with CAH in Australia has been arrested for the death of another person".

Susan's friend said Kathleen helped out her younger sibling and her friends whenever they needed it. "She was never angry or harsh towards any of us," she said. "Every time I saw her, she had a huge smile on her face and was very friendly to us."

Kathleen wrote of her desire to work in child counselling after university.

Her Facebook page also lists her interests as sport, drinking, cooking, social justice causes and hanging out with friends.

Susan was dedicated to her studies, her friend said. "She was the smartest person I've ever met."