Brenda Lin didn't realise her uncle's abuse was relevant to her family's murder trial.

Brenda Lin has recalled how she did not think the abuse she suffered at the hands of her uncle, Robert Xie, was “relevant” to the investigation into the murder of her mother, father, aunty and two brothers.

In July 2009, Lin was just 15 years old and enjoying a school excursion to New Caledonia with her friends when her family was killed in their Sydney home.

It was two years before Robert Xie was arrested for the murders of Min Lin, 45, his wife Lily, 43, the couple’s sons Henry, 12, and Terry, 9, and Lily’s 39-year-old sister Irene.

However, in the immediate aftermath of the tragedy, the teenager was unwittingly sent to live with her family's killer — and was regularly sexually abused by him during this time.

In an interview with Channel Seven's Sunday Night, Lin described the initial disbelief and then "eye-opening" shock she felt at learning he was the prime suspect.

"My aunt and uncle, they were the substitute family I had... They were the next best thing to my family," she said.

"He was definitely not a murderer in my eyes. He was an uncle. A family guy, that was it."

Lin may not have been suspicious of her uncle in the years before his arrest, but others were.

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Susan Bridge, the principal at Brenda's high school, recounted seeing a "potentially dangerous" side to Xie during a disturbing encounter.

"He so physically very close and [his] face only this far away, speaking, loudly, rapidly in Chinese. Brenda with her head bowed down, tears falling down. It was clear he was trying to get her to agree to something," Ms Bridge told reporter Melissa Doyle.

"I did feel that I had seen a side of Robert that was a potentially dangerous side."

The educator said she told her husband that night, "I think I've spent all day with a murderer".

Ms Bridge shared her concerns with police, who were already building a case against Xie.

As they collected evidence, investigators did not know Xie was continuing to carry out crimes against the remaining Lin family member.

Lin confirmed the abuse she suffered to Doyle, but did not wish to go into detail.

"Yes he did, and that's something that I'm very private about," she said.

"It's something that at this point in time I don't feel comfortable talking about as well, so I hope people can respect that and respect my privacy and know at one point in time in the future I maybe will be able to talk about it."

Lin did, however, attempt to describe her vulnerability at the time.

"I think at the times after the murders he... as a teenager myself I was very insecure, very... I didn't know what I was doing. And he knew that. He read it very well."

The then-teenager told her lawyer about the abuse during Xie's first trial, revealing the inappropriate touching had begun before the murders.

"Brenda revealed something that she didn't think was relevant to the murder trial, but which I thought was. And so I think the trial judge had no choice, but to start the trial again," the lawyer said.

It was during the second trial that prosecutors argued that a sexual motive drove Xie to kill the Lin family in order to gain unfettered access to Brenda.

After the judge in the second trial fell ill and the third related in a hung jury, a fourth trial finally saw Robert Xie found guilty of the murders and sentenced to life in prison.

The judge referenced the sexual motive in sentencing. However, Lin said she could never be sure if that was the case.

"For a moment I did think that, but I also don't think that something like this would warrant him to kill five people," she said.

"I don't know what goes through his mind and I can't be sure and I don't think I ever will be sure about why he was motivated to do what he did. "

The now 22-year-old is moving on with her life with the support of family and friends and says she is heartened by the well-wishes of strangers.

"It took amazing people in the wider community who have done amazing things... I've seen the good in people," she said.