Costco store manager Roger Cooper, 41, and ex-soldier David Cooper, 39, both deny killing Sameena Imam, 34.
Her body was found buried at an allotment in Leicester in January after allegedly been killed with chloroform.
Ms Imam had been having a two-year affair with Roger Cooper and issued him with an ultimatum to leave his partner.
Roger Cooper, of Coventry, denies murdering his colleague after driving her from Coventry to Leicester on December 24, claiming he dropped her off at a supermarket following an argument.
His brother, of Leicester, has admitted burying Ms Imam's body but denies doing anything to cause her death.
The prosecution claim Roger Cooper wanted Ms Imam out of his life as he conducted three relationships, and enlisted the help of his brother to "plan and execute" the alleged murder.
A trial at Birmingham Crown Court, which began in August, was told by a toxicologist that several metallic elements were found in the body of Ms Imam.
She had worked at Costco outlets in Cardiff, Coventry, Southampton and Bristol.
A jury trying two brothers accused of murdering a cash-and-carry manager from Cardiff has retired to consider its verdicts at Birmingham Crown Court.