Renee and Andrew MacRae, who was three, vanished on 12 November 1976.
Mrs MacRae's burned-out BMW was discovered that night in a lay-by on the A9 south of Inverness.
In statement, the family said it was "collectively heartbroken" the pair remained missing 40 years on but were still hopeful of finding answers.
Police Scotland said an investigation into their disappearance remained ongoing.
On the evening of her disappearance Mrs MacRae, 36, had set off to meet her lover Bill McDowell in Perth but he insisted they never met.
There has been speculation that Renee MacRae and her son were murdered and their bodies buried at either a quarry or at construction works for the A9.
In 2004, police searched nearby Dalmagarry quarry but no bodies were found.
Two years later a report naming a suspect who may have killed the pair was sent to prosecutors but they decided there was insufficient evidence to take action.
It has been reported recently that "an anomaly" has been detected by ground penetrating radar in the foundations of a bridge near the lay-by where the car was found.
However, Police Scotland said its inquiries indicated construction work did not start in this area until some time after Mrs MacRae's disappearance.
The force said it would nonetheless liaise with contractors involved in the current A9 upgrade in an effort to explain the radar anomaly.
In their newly released statement, the family said: "Forty years have passed since the disappearance of Renee and Andrew and as a family we remain collectively heartbroken to have lost a much loved and cherished mother, sister, brother and friend to many.
"We cannot give up hope that somebody holds information which could help lead us to the answers as to what happened to our beloved Renee and Andrew.
"Our message is it is never too late. We are confident these answers will come from the local community and as a family we urge that person to come forward - until such time the person who caused harm to Renee and Andrew will continue to escape justice and we will be without closure."
Det Supt Jim Smith, of Police Scotland's Major Investigations Team North, said: "As in all cases such as these, there is a family quite rightly seeking answers and closure.
"We are determined to do all we can to find those answers, and to that end continue to maintain contact with the family of Christine MacRae and Andrew MacRae as the years go on.
"The passage of time is no barrier and we continue to urge anyone who may have information that could assist the investigation to come forward."
The family of an Inverness mother and her young son have said they are heartbroken that their disappearance remains unexplained 40 years on.