Russell Hill's wife believed Carol Clay was her husband's cousin for years before it was revealed the pair were having an affair, a jury has been told.
Mr Hill, 74, and Mrs Clay, 73, were camping together in Victoria's Wonnangatta Valley when they disappeared in March 2020.
Greg Lynn, who was a Jetstar captain at the time, is accused of killing the couple and dumping their remains in bushland.
He has pleaded not guilty to two counts of murder and is on trial at the Supreme Court in Melbourne.
Mr Hill's wife Robyn appeared as a witness on Tuesday, where she told the jury she first met Mrs Clay about 50 years ago.
Mr Hill told her Mrs Clay was his first cousin, she said.
The Hills went on trips with the Clays and Mrs Hill said she noticed her husband and Mrs Clay going on walks together, which she thought was "strange", she said.
In the early 2000s, a neighbour confronted Mr Hill about the affair and gave him an ultimatum.
Mrs Hill said her husband admitted his relationship with Mrs Clay, but said it was over and he wouldn't see her again.
She believed her husband was travelling alone to the Wonnangatta Valley, in March 2020, and helped him pack items including antidepressants and cans of bourbon, the jury was told.
After not hearing him on his usual radio broadcast for three days, Mrs Hill became worried and contacted police.
She said Mr Hill had loved the valley and seemed happy to be going camping.
The 74-year-old was fit and healthy, although he was starting to "walk and think slow".
"I just thought he was getting old," Mrs Hill told the jury.
Her daughter Deborah also gave evidence on Tuesday, about a time Mr Hill used a shotgun for duck hunting in the mid-1980s.
But she never saw him use a firearm again as he believed they were unsafe to have around, she said.
It's alleged Mr Hill and Mrs Clay died on March 20 after an argument between Lynn and Mr Hill at Bucks Camp.
Mr Hill had threatened to send his drone footage of Lynn deer hunting near the camp to police, defence barrister Dermot Dann KC said.
He claimed a scuffle broke out after Mr Hill took a gun from Lynn's vehicle and Mrs Clay was accidentally shot by Mr Hill.
The barrister said Mr Hill came at Lynn with a knife and was accidentally stabbed in the chest as Lynn defended himself.
Lynn then burned Mrs Clay and Mr Hill's campsite, put their bodies in his trailer and dumped them on the remote Union Spur Track, near Dargo.
Footage and photos shown to the jury on Tuesday showed a burned campsite and a number of items, including the victims' wallets, a high frequency radio and a mobile phone, inside Mr Hill's Landcruiser.
Earlier on Tuesday, the jury was reduced from 15 to 14 people after a juror was discharged because of illness.
Justice Michael Croucher said it was better to continue on rather than wait and see if her health improved later in the week.
The trial continues.
