The TV show revealed how the four siblings recreated the crime scene, which included gathering and reviewing evidence, and talking to neighbours.
Nadine had been in an alleged volatile relationship.
Her former partner has strenuously denied any involvement in her death, Sunday Night reported.
On the day of her death she had an appointment with a lawyer and had made plans to return with her young daughter to their family in Queensland.
Her body was found the following day in the shower.
As her death was treated as a suicide, police did no forensics work and took no fingerprints or DNA from the scene.
As part of their private investigations the siblings found a message tucked in behind the original suicide note saying, "He did it".
Those same words were also found just a few months ago - etched on a low tile in the bathroom by new residents of Nadine's former apartment.
Sunday Night reported they also could not locate the brown dress their sister was wearing on the day she died.
NSW Police were not available for comment yesterday.