Lady Markby plays the role of the gossipy aunt. If you want the world to know your business, tell it to her; it's clear that nothing ever ends with her. In the play, she represents the pre-modern woman. She counsels Mabel: "Nothing is so dangerous as being too modern. One is apt to grow old-fashioned quite suddenly" (2.213).
It's also interesting that she is the person who introduces Mrs. Cheveley. Decidedly unaware of the tools of warfare in modern life, she doesn't have a clue that she's starting a skirmish.