The appeal comes ahead of tomorrow's 20th anniversary of the uncovering of the horrific sex murders which Britain's worst female serial killer carried out with her husband, Fred.
The level of depravity and barbarity with which the Wests went about their murder spree in Cromwell Street in Gloucester shocked the nation. Rose West was convicted of 10 murders in 1995 and is now serving a life sentence.
Her husband Fred was charged with 12 murders but hanged himself in prison before his trial. The victims were tortured over lengthy periods and raped before being killed.
Mary Bastholm was 15 when she went missing on her way to catch a bus to visit her boyfriend in the city in 1968.
Her brother, Peter Bastholm, has now called on West to divulge what happened to his sister. Bastholm, 69, believes Mary was an early victim of Fred West. Mary's body has never been found. Bastholm said Rose West was probably the only person who knew about his sister's disappearance.
"I think Rose knows exactly what happened to Mary because I assume she and Fred discussed it between themselves. Whether she will ever disclose it, I really don't know," Bastholm said. It would be a final act to find Mary's body and lay it to rest after all these years, after all we have gone through."
Mary vanished in 1968 - a time when Fred West, who often did jobs in the street where she worked as a waitress in a cafe, was abducting girls in Gloucester. Her body was not found during the excavation of the West's home which began on February 24, 1994. The property became known as the House of Horrors.
However, police investigating the murders did find evidence that suggested a link between Mary and Fred West.
Bastholm said: "My wife and I went down to the police station to view a lot of things that had been taken from the Wests' house. My wife picked out a silver locket on a chain and she said that was Mary's because she wore it as a bridesmaid at our wedding.
"We had a photo of it, but because it was a black-and-white picture the police said they could not tell if the locket was silver or gold, so nothing came of it."
Fred West is also said to have confessed to killing Mary in conversation with his son, Stephen, but never admitted it to police.
Tony Butler, former chief constable of Gloucestershire Police, said: "I don't know if Rose West will feel at some point that she has some moral obligation to help the relatives of people I don't know."
But her solicitor, Leo Goatley, says he believes West would reveal what happened to Mary if she knew.
"If Rose West was asked now about what happened to Mary Bastholm, I think if she knew, she would say so," Goatley said.