THREE sisters have made a public appeal to help find out what happened to their sister who disappeared more than 40 years ago.

Mary Duncan - step-daughter of Bonhill child abuser Norman Duncan - left home at 51 Third Avenue in Bonhill on March 19, 1976, aged 17, without her young child and has never been seen or heard from again.

Now her three sisters, Debbie Renwick, Mandy Duncan and Marion McFarlane, have stepped forward to make a public plea to residents of Dumbarton and the Vale on what happened to Mary.

Cops leading the investigation into Mary's disappearance have spoken to Norman Duncan, who was convicted in September 2016 of abusing Debbie and Mandy.

He was jailed for five years and both women last year waived their right to lifelong anonymity as victims of sexual abuse.

Detectives believe it is local knowledge that could be key to find Mary, particularly former workers of the Vale of Leven Hospital or people who were around the hospital at the time.

At an emotional press conference, Debbie, 56, said: “Somebody must know something. Somebody must know what happened to her and we hope that person will come forward now.

“We are miserable without her. We have missed her every year. She was our elder sister.”

She added: “We just want to bring her home. Please bring her back.”

Marion, 59, who has never spoken previously, said any information from the public could be “the missing piece of the jigsaw”.

She said: “Mary matters a great deal even now. She would have been 60 next year. We have missed a whole generation.”

Mary’s disappearance was considered completely out of character, particularly leaving behind her 13-month-old baby Laura, who later died of natural causes in October 1976.

Detective Superintendent Calum Young said Mary was known to have frequented the grounds of the Vale of Leven Hospital and they were particularly keen to hear from former staff or residents who may have seen Mary.

Det Supt Young said Norman Duncan was just one of a number of lines of enquiry and they were keeping an open mind about the investigation, which remains a missing person case.

He said: “At the age of 17, Mary was a devoted mother to a baby girl called Laura. Her family say Mary absolutely adored her daughter and that her disappearance, and leaving Laura behind, was completely out of character.

“Mary left home with no money and she has never claimed benefits, paid tax or national insurance. Her family strongly doubt that she would have had the means or life skills to start a new life and live independently.

“Whilst there is no evidence to suggest that Mary came to any harm, due to the fact that she has not been seen or heard from in 41 years, we can’t rule this out.

“Mary’s family were left completely devastated by her disappearance, a pain which lives with them to this day, and it is vital that we get to the bottom of what happened to their sister and provide them with some closure.

“I would ask anyone who knew Mary or her family in the years before her disappearance to please get in touch to help us piece together the details of her life around the time she went missing.

“No matter how insignificant you think your help may be, let us be the judge of that because even the slightest detail could be important. Even if you spoke to police back in 1976, there’s been a huge passage of time and we would like to speak to you again.

“Some of our enquiries have focussed on the Vale of Leven Hospital and I would appeal to anyone who worked there in 1976 to come forward as they may be able to assist with our investigation.

“I believe the answer to what happened to Mary lies within the local community. Mary’s family have waited long enough for answers. Please, if you have any information at all, contact Dumbarton Police Station on 101.”

Mary was last seen wearing a navy skirt, black platform shoes, green hooded zipper, and a black and white dogstooth coat.