Jenny Cupit, 24, began an affair with computer consultant Christopher Linaker after they met at a local amateur operatic and dramatic society in Warrington, Cheshire.
Their relationship became increasingly intense, involving group sex with other partners but when Mr Linaker refused to end his marriage, Mrs Cupit took her revenge on his wife Kathy, 33, the jury at Chester Crown Court was told.
Alex Carlile QC, for the prosecution, said Mrs Cupit armed herself with a kitchen knife bought from Asda, went round to the Linakar's home in Warrington and bludgeoned Mrs Linaker with a decorative blue bottle before fatally stabbing her. Former hairdresser Mrs Cupit, of Orford, in Warrington,denies murdering mother-of-two Kathy Linaker on 17 April last year.
"The prosecution say that whilst at Kathy Linaker's home and in the presence of Kathy's four-month-old baby, Holly, the defendant hit her repeatedly and stabbed her to death," said Mr Carlile.
The court heard how the Cupit and Linaker families met while members of Warrington's Centenary Operatic and Dramatic Society. Mr Carlile said: "The defendant and Chris Linaker began an affair. The wives, the defendant and Kathy Linaker had been friends and they continued to be so."
The court was told that just days before Mrs Linaker's death Mrs Cupit had tried to persuade Mr Linaker to run away with her to Canada. Mr Carlile said: "He (Mr Linaker) thought that the defendant might harm herself if he ended the affair. He had no intention to leave his wife."
A post mortem examination found evidence of repeated stabbing and heavy blows - "a sustained attack involving at least three weapons".
Mr Carlile told the court that the knife bought by Cupit and a second knife taken from the Linaker kitchen drawer were found by the victim's body. The blue bottle was found concealed in a bin bag at the side of the house.
The prosecution added:"If she couldn't have Chris Linaker for herself then he wasn't going to have Kathy.
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