The murderer was Midori Ozaki. Some years before the murders, Rika and Shiraki stole the sevret ikebana techniques that Midori's beloved older sister invented and blacklisted her from the ikebana circles; Miss Ozaki fell in despair and commited suicide by poisoning several bunches of flowers in her room and then performing her signature trick to make them open at the same time, releasing the poison that took her life away. A little later, Midori found her sister's body.
Midori searched for info on who stole her sister's trademarks and soon found Rika, who was claiming she was the true creator. So she became Rika's manager and staged a gambit to kill both her and Shiraki. She first drugged Shiraki ans strangled him, then poisoned Rika's stage flowers and got ready. Had said gambit worked, both Rika and Midori would've died; Rika would've been "punished" for indirectly causing Miss Ozaki's death, and Midori would escape punishment and be "reunited" with her dead sister in death.
