Yesterday an inquest into her disappearance heard she could have been planning to run away to escape an allegedly abusive home.
Helen had been sharing a house at Marrickville, in Sydney's inner west, with her six siblings for only a month after her parents' relationship broke down and her mother Voula Karipidis was awarded custody, the inquest was told.
Sergeant George Lolis, assisting the coroner, said police had investigated and ruled out various scenarios including a family member or stranger killing her, or that she had run away.
But Anne Martin, a teacher who formed a "close relationship" with Helen's older sister Maria after the disappearance, told the inquest she heard disturbing allegations of abuse.
"(Maria) told me their father was fiercely violent at times towards them ... they were both very afraid of (him)," she said.
Maria, who was 12 at the time her sister vanished, and who committed suicide when aged 18, claimed Helen was "preparing to run away and had wrapped her Christmas presents early", Ms Martin said.
"(Maria) felt responsible because she said she should have gone with her that night."
The girls' father Theo Karipidis and mother, who were in court for the inquest, suggested Maria had been prone to "making things up", but Ms Martin said she had always found her to be truthful.
The inquest continues.