Bittersweet Motives

Former reporter Serena "Rena" Manchester has transitioned into her new career as a private detective. The Clayton Matthews Law firm sends a few investigations her way that pay the bills, though the work is hardly exciting. However, a shift occurs one day while she is taking photos of a prominent businessman, Edwin Peters, with his young mistress. When the woman shows up dead, Serena has no doubt that Edwin Peters is the prime suspect.
A monkey wrench is thrown at her theory when Serena realizes her client, Mrs. Judith Peters, has just as much motive to kill the young woman. Rena is going to need her hard-nosed tactics to find a murderer because the suspects keep piling up.
Which one of these people succeeded in killing the troubled woman?