The jury took more than six hours to find Michelle Taylor, 21, and her sister Lisa, 19, of Forest Hill, south-east London, guilty of murdering Mrs Shaughnessy, 21, on 3 June 1991. She had been stabbed 54 times.
After the jury returned its unanimous verdict, Mr Justice Blofeld told the sisters he was passing the sentence prescribed by law. 'You have been found guilty of this terrible crime of killing Alison Shaughnessy, the wife of John Shaughnessy, whose life was all before her and whose life is now no more.'
Michelle, the lover of Mrs Shaughnessy's husband, clutched a crucifix and wept as she was led to the cells by three prison officers. Her sister, Lisa, had to be supported. She will begin her sentence in youth custody.
Michael Holmes, their solicitor, said the sisters would appeal against the verdict. They have always protested their innocence. 'I have seen them in the cells and they are in tears and very upset as you would expect them to be. They cannot believe the verdict,' he said.
After the verdict, John Shaughnessy, Alison's husband, embraced her family and said that justice had been done. 'Alison can now rest in peace now we know that those two people have been sent to prison.'
Breda Blackmore, Mrs Shaughnessy's mother, added that the family was very pleased. 'No sentence will ever atone for Alison's death. It doesn't matter how many years they serve. They will be branded murderers for the rest of their lives,' she said.
After the trial Det Supt Chris Burke described the verdict as 'brilliant'. He said the sisters had 'coldly, calculatedly and wickedly manipulated their way into a young bride's home and brutally stabbed her to death 54 times'.
Derek and Ann Taylor, the sisters' parents, left court weeping and refused to discuss the verdict.
Michelle was infatuated with Mr Shaughnessy but knew he would never leave his wife. She believed the only solution was murder.
In her diary she dreamed about life without Mrs Shaughnessy: 'If she never existed then maybe I could give everything I wanted to the man I love.' Alison Shaughnessy was killed after she arrived home from work on 3 June. Michelle stabbed her in a 'frenzied attack' with a 5in knife while Lisa encouraged and supported her.
The sisters then drove to the Churchill clinic, in Lambeth, to create an alibi. They persuaded a friend, Jeanette Tapp, to tell the police they had been with her all afternoon. She agreed, but withdrew her statements after being charged with conspiracy.
After helping Mr Shaughnessy to arrange the clinic's flowers, Michelle drove him home. When Michelle saw Mrs Shaughnessy lying at the top of the stairs, she 'affected horror and shock'.
Two months after the murder the sisters were arrested. Police had found Lisa's fingerprints on the front door of the flat. But
she told the police that she had never been there. Lisa refused to say anything except, 'I haven't done it'.
Although Michelle denied murdering Mrs Shaughnessy she told the police that she hated her and was jealous of her. She added that she felt sorry for any woman whose husband wanted to be with someone else.
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