What is Mrs. Catherick's secret in the novel, The Woman in White?

Philip Fairie impregnates a young maid while staying at Varneck Hall but does not marry her. Instead, he marries a respectable woman and they later have a daughter named Laura. The maid, however, then rushes into marriage to a church clerk named Catherick, and they live in Welmingham. Their daughter is born and named Anne Catherick. Anne is therefore a rightful heir to the Fairlie estate, but she does not know it and she looks remarkably like Philip Fairlie's daughter Laura. The fact that Anne is the daughter of Philip Fairlie is Mrs. Catherick's secret.