Isobel Flemming, played by Mia Kirshner, is introduced in the first season of The Vampire Diaries. It is revealed that she is the main character Elena's mother. Her story arc spans the first two seasons of the series.
She clearly cares deeply for her former husband Alaric Saltzman, and never talks about having relationships with women. However, she is shown enjoying time with a scantily clad man and woman. The woman is also shown with a bite mark on her inner thigh, implying that Isobel had been snacking on her as well as the man.
When John Gilbert walks in on the three of them, he doesn't seem at all surprised by the fact that they are all in the room.