William MacDowell, of Penrith in Cumbria, is accused of murdering Renee MacRae, 36, and their son Andrew MacRae in November 1976, and is on trial at the High Court in Inverness.
On Thursday, Morag Govans told the jury of her sister Mrs MacRae’s dedication to her children.
She said: “Renee was a devoted mum to her two boys. She adored her boys. That’s what she lived for.”
The 84-year-old told the trial she and her sister used to phone each other every Sunday, and her concerns grew for her welfare when detectives visited her home after the disappearance.
Ms Govans said: “I was very worried. I knew something dreadful had happened to Renee and Andrew. I knew Renee would never have gone away and left her other son Gordon behind.
“I know Renee wouldn’t have put me through that. She would have contacted me if she had gone anywhere.”
MacDowell is charged with assaulting Mrs MacRae and their three-year-old son at the Dalmagarry layby on the A9 trunk road south of Inverness, or elsewhere, by means unknown, and as a result murdering them.
The 80-year-old is also charged with disposing of their bodies and belongings by means unknown.
He denies the charges against him, and has lodged two special defences of incrimination and alibi.
Before Mrs MacRae’s burnt-out BMW was found on November 12, 1976 in the layby, she had told her estranged husband Gordon MacRae she was going to her sister’s house.
But the court was told that Valerie Steventon, one of her closest friends, had a conflicting story.
Mrs Govans told the trial: “She told me she was away with Andrew’s father for the weekend.
“I phoned Gordon MacRae and told him what Valarie told me and I told the police.”
She said after the disappearance she called MacDowell, but his wife answered and said he did not want to speak to her.
She later went to his house but did not get an answer, the court was told.
MacDowell was the father of Andrew, but the court was told Mrs MacRae kept his identity a secret from others.
Sheila Fraser, 80, knew Mrs MacRae through her husband and was visited by her in the weeks before her disappearance.
MacDowell’s defence of incrimination and alibi has stated he was with others and then later at home on the evening of the disappearance.
He claims Mr MacRae, along with others, committed the alleged crimes.
Part of his alibi is that he went to the Mercury Motor Inn in Inverness and was in the company of three men, including John Davenport, until about 7.15pm that night.
Mr Davenport told the court that MacDowell had told the group present he was to meet his wife at 7pm that evening, and made the reference a couple of times.
The court also heard a statement from the late Charles MacRae, the brother of Mr MacRae, who told officers she appeared “off” her husband and that pills and drink had been found in her home.
The trial continues.