Renee MacRae, 36, and three-year-old Andrew disappeared after leaving their home near Inverness on 12 November 1976.
Her BMW was found burnt-out a few miles south of the city that day. The case remains unsolved.
Speaking on the anniversary of their disappearance, Mrs MacRae’s sister Morag Govans said: “Not a day goes past that both are not in my thoughts.
“Even after the passage of time I believe that there are people who know what happened to Renee and Andrew, and I will never give up hope of getting justice for them both.
“I appeal for anyone with any information to come forward and for others to examine their conscience to end 42 years of pain and anguish to allow us to grieve for Renee and Andrew.”
Detectives said the time that has passed was “no barrier to justice” and the latest forensic technology and investigative techniques were being applied to find out what happened to the mother and son.
Officers from Police Scotland’s major investigations team and marine investigation unit recently spent several days at Leanach Quarry near Culloden as part of the inquiry.
Andrew’s pram, believed to have been with the pair when they went missing, has never been recovered.
It is described as a Silver Cross Carnival collapsible model, with a chrome-plated frame and a blue canvas cover.
Ms Govans said: “Andrew was a mischievous and fun-loving wee boy. It is impossible to take in that his life was brought so cruelly to an end at such a young age.
“He would have turned 45 in October and I often wonder what he would have been doing now with the life that he was never given the chance to build.
“Renee was a compassionate and caring mother. Both Andrew and his elder brother Gordon were her life. She adored them and was so proud of her boys.
“Gordon was only eight and has lost the love and guidance of his mum and brother.”
Detective Inspector Brian Geddes said: “Our goals are to bring a person, or persons, to justice and to locate the remains of Renee and Andrew in order to bring some form of closure to Morag and the rest of the family.
“The chances are that person or persons responsible for the murders of Renee and Andrew will have enjoyed a life, something which was denied to them.
“With that in mind I would again appeal to anyone who has information, even if they are not sure if it is important, to come forward and contact us.”
Mr Geddes said he was keen to speak to the anonymous author of a letter sent to police in 1982 regarding information surrounding activity at an unnamed quarry on the day Mrs MacRae and her son went missing.
He said: “Forty-two years is undeniably a long time, however it is certainly no barrier to justice. Someone out there knows what happened to Renee and Andrew, and I hope this appeal today serves as a reminder that it is never too late to come forward with information.”