Investigators spying on Robert Xie while gathering evidence against the mass-murderer were faced with a dilemma after watching him 'uncomfortably' touch his niece.

Should they tell the teenage girl that her uncle was the main suspect in her family's killings?

The revelation on Sunday that cops observed Xie touch his niece, Brenda Lin, in an 'uncomfortable' way was especially distressing in light of the recently revealed sexual motive for the slayings.

Earlier this week, it was reported that prosecutors argued Xie killed Ms Lin's parents, brothers and aunt so that he could have unfettered access to the teenage girl - whom he has been accused of repeatedly sexually assaulting after she moved into his house following the murders.

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Investigators spying on Robert Xie while gathering evidence against the mass-murderer observed him 'uncomfortably' touch his niece, Brenda Lin (pictured)

The revelation on Sunday that cops observed Xie touch his niece, Brenda Lin, in an 'uncomfortable' way was especially distressing in light of the recently revealed sexual motive for the slayings

'To this day, my biggest regret was not hugging (my father)': While Ms Lin was on the week-long trip to New Caledonia, her father Min (right) was brutally murdered

Police observed the 'uncomfortable' touching by hiding 'pinhole' cameras in Xie's home, the Daily Telegraph reported on Sunday.

The cameras were installed after Xie, 53, became a main suspect in the July 2009 slayings.

Among several instances of touching captured by the hidden cameras, Xie was observed laying a hand on Ms Lin's shoulder while she was seated at a desk and studying.

Ms Lin was only 15 years old and had been abroad on a school trip when her family was killed.

While police said the touching they observed didn't amount to molestation, Ms Lin has accused her uncle of sexual assault. However, Xie has not been charged with sexual offences.

Xie was last week sentenced to five lifetimes in prison for the murders of Ms Lin's parents Min and Lily, aged 45 and 43, her two younger brothers Henry, 12, and Terry, 9, and her auntie Irene, 39.

Across four murder trials spanning seven years Brenda's identity was kept hidden, but that suppression order was lifted when she agreed to appear in an exclusive interview with Channel Seven's Sunday Night program, Fairfax reported.

Brenda Lin's parents Min, 45, and Lily, 43 were killed after being bludgeoned with a 'hammer-like object' by Robert Xie

Ms Lin, who is now in her 20s, will break her silence and reveal the 'sexual motive' behind her uncle's horrific crimes in the tell-all interview.

'I do not even know how to begin to express how the murder of my immediate family have impacted my life – there are not enough words to describe the pain and suffering caused me and those around me,' Ms Lin said in her statement to court.

She also spoke about the last time she saw her father.

Shortly before the murders, while about to board a plane for New Caledonia to go on a week-long school excursion, she chose not to hug or talk to her dad in front of her classmates.

She had no idea that while she was gone, her entire family - including both parents, two younger brothers and auntie - would be murdered by Xie.

'Being a prideful teenager I did not say anything to my father, I just stood there awkwardly and thought to myself 'It is just going to be a week, I am going to see them again really soon',' Ms Lin said in court during Xie's sentencing last week.

'To this day, my biggest regret was not hugging (my father) and telling him I loved him, to say thank you for being an amazingly loving and caring parent.'

'It has been seven-and-a-half years since I have lost my family. That's seven-and-a-half years without a loving mother, seven-and-a-half years without a loving father.

A furious struggle took place in the bedroom of her two young brothers Henry (left) , 12, and Terry (right), 9 - but both were killed by their sinister uncle

Irene - Brenda Lin's auntie - was the fifth member of the family to suffer a horrendous fate at Xie's hands

'Seven-and-a-half years without two exuberant brothers who were my best and closest friends and seven-and-a-half years without an extremely kind aunt.

'In this time I have finished my HSC, was accepted into uni, got my first part-time job and learnt to drive. But I have achieved all these things without my family beside me.

'These inherently happy moments are now at most bittersweet, they have now become a painful reminder of the family I have lost and I will never see again.'

During the trial police revealed that the killer appeared to be familiar with the layout of the home and chose to enter Ms Lin's room, pointing them towards Xie as a suspect.

The court also heard that Xie touched Ms Lin inappropriately and the assaults became more frequent after she had to move into his home after the killings.

Xie offered to adopt Ms Lin immediately after the murders, and allegedly assaulted Ms Lin up until he was charged with her family's murders in 2011, the court heard.