Melbourne man Adam Reid, 38, died when the Cessna Grand Caravan operated by his employer Tropicair crashed in PNG's Gulf province on Monday.
PNG nationals Stanley Kovi and Joyce Kori were also killed in the crash. Six other passengers and the pilot survived.
Mr Reid's older brother Matthew was killed in a head-on collision with a semi-trailer in New South Wales earlier this year.
Their cousin Noel Nealon says the family is devastated, especially their mother Cheryl.
"What do you say to a mother who lost two of her three children that she cherished?" Mr Nealon said.
"All she wanted to be was a mother and to lose them is such a tragedy."
Mr Reid was an observer on the flight and had recently relocated from Melbourne to PNG to get more pilot hours.
Mr Nealon says his cousin dreamt of becoming a pilot for the Royal Flying Doctor Service.
"He wasn't even flying. He was just an air observer in the plane so whether he needed to be there or not, but unfortunately it happened," Mr Nealon said.
Pilot issued mayday call
The CEO of PNG's Accident Investigation Commission, David Inau, says the pilot made a forced landing after experiencing engine trouble.
"The pilot did put out a mayday call and did state that he was having engine problems," he said.
Investigators are today trying to move the submerged wreckage of the plane onto a barge to retrieve the bodies of those killed in the crash.
Tropicair has suspended its operations while an investigation is carried out.