Brother of Maria Lutz who died in 'murder-suicide' with her family is demanding answers.

A week on from the murder-suicide of Maria Claudia Lutz, Fernando Manrique and their two children Elisa and Martin, answers as to what went wrong for the Sydney family are being desperately sought by Maria’s brother.

Juan Lutz Pena arrived in Australia from the family’s native Colombia days after the tragedy and is now trying to understand what lead to a crime that has rocked the northern Sydney community.

Speaking to the Daily Telegraph, a source close to the family said Juan "was very close to Maria and, as a father himself, adored Elisa and Martin. He longed for them all to come back to Colombia to be close to family."

 The family were discovered inside their Davidson home along with the family dog last week, having been gassed.

Within days of the discovery, neighbours reported seeing 44-year-old Manrique working on the roof of their house. It is now known he was installing an intricate pipe system that would later turn the house into a giant gas chamber.

While friends described Lutz as eternally positive and optimistic, it now appears coping with the needs of her two severely autistic children and the breakdown of her marriage was taking its toll and is something Juan feels was not managed adequately.

The source continued, "Juan is deeply upset by the death of his sister and wants to know why the relationship between Family and Community Services broke down.

"Maria was so anxious and depressed that if [the Family of Community and Services] called to change an appointment or cancel [then] it threw her."

"She would ring them sometimes nine times a day to confirm they had cancelled or changed things - or to ask for extra help," the source continued.

"She felt completely alone and was a broken woman and depressed."

Juan and his parents, Alicia and Ernesto, are now left to plan the funeral of their much-loved Maria and her children.