Det Supt Chris Burke, who led the murder inquiry, described the sisters as 'clever and calculating killers'. He said they had clearly planned every aspect of the murder of Alison Shaughnessy. 'Michelle made sure she found the body, picked it up and so destroying any forensic evidence.'
Mrs Shaughnessy, a bank clerk, of Battersea, south-west London was devoted to her husband, John. They met at an Irish public house in 1986, he was her first boyfriend. Four years later she married him in Piltown, Co Kilkenny; a year later she was buried there. 'Her wedding day was the biggest day in her life,' her mother, Breda Blackmore, said.
Michelle Taylor, Mr Shaughnessy's lover, stabbed Mrs Shaughnessy 54 times, in a 'frenzied attack ' on 3 June 1991, while Lisa encouraged and supported her. The murder was the realisation of Michelle's 'dream solution'. She described her hatred and jealousy of Mrs Shaughnessy in her diary. 'I hate Alison, the unwashed bitch. My dream solution would be for Alison to disappear as if she never existed and then maybe I could give everything I want to the man I love.'
Two days after the murder Mr Shaughnessy appealed for witnesses. He said the killer was not human. The police found a five-inch knife concealed inside a mop in a cleaning van belonging to the girls' father Derek Taylor, but after extensive forensic tests nothing was found. Mr Taylor was later convicted of possessing an offensive weapon.
Michelle met John Shaughnessy in 1989 at the Churchill clinic in Lambeth, south London, where they both worked. Initially they were just 'good friends' but Mr Shaughnessy admitted that he soon 'developed an attraction for Michelle'. Although he was engaged to Alison, Mr Shaughnessy began a sexual relationship with Michelle. She was a virgin and he asked her to go on the Pill.
Six months later he admitted he was engaged and Michelle ended the relationship. But it resumed several months before Mr Shaughnessy's wedding in June 1990.
Michelle organised and paid for his stag night and Mr Shaughnessy invited her to the wedding in Ireland as 'his guest' and paid for her flight and hotel. Michelle said they slept together on the morning of his wedding, but Mr Shaughnessy denies this.
After the wedding Mrs Shaughnessy moved into her husband's room at the Churchill clinic, it was two doors away from Michelle's room. He continued to sleep with both of them. Breda Blackmore believes her daughter never suspected that Mr Shaughnessy was having an affair with Michelle. 'John was Alison's only boyfriend, he was her life. As far as we knew, she was his life as well,' she said.
The couple moved to Battersea in January 1991 and Michelle helped them move. She now began to feel used and jealous of Mrs Shaughnessy. She knew Mr Shaughnessy would never leave his wife and that they were planning to start a family. For her the only solution was murder. On 3 June, the two sisters waited for Mrs Shaughnessy to arrive home. At about 5.45pm they knocked on the door and she let them in. As they climbed the stairs, Michelle attacked her with a five-inch knife. Professor Rufus Compton, a pathologist, said the murder could only have taken 2 to 3 minutes.
As the sisters left the flat they were seen by Dr Michael Unsworth-White. He later picked Lisa out in an idenity parade.
After helping Mr Shaughnessy to arrange the clinic's flowers, Michelle drove him home. They found Mrs Shaughnessy lying at the top of the stairs. Michelle picked her up, eliminating any possibility of testing her clothes, car or hands for forensic evidence.
The sisters persuaded Jeanette Tapp to give them a false alibi. She agreed because she 'trusted' them. Four days after the murder Miss Tapp made two statements claiming that they had all been in her room from 5.15pm. But a month later Miss Tapp was arrested for conspiring to murder Mrs Shaughnessy and immediately withdrew the statements.
After Lisa's fingerprint was found on the front door of the flat, the sisters were arrested. As she left her home under police escort, Lisa shouted to her mother, 'Mum I didn't do anything.' Michelle protested her innocence and insisted the affair was over months before the murder. But police believe the relationship only ended with her arrest.