The Duke's Children by Anthony Trollope

In the last of the six Palliser novels, the sudden death of his wife, Lady Glencora, leaves Plantagenet Palliser, the Duke of Omnium, finding himself in charge of his three children. The eldest, Lord Silverbridge, has recently been expelled from Oxford; his younger brother, Gerald, is about to enter Cambridge; and the youngest, nineteen-year old Lady Mary, has imprudently formed an attachment to Francis Tregear, who, while certainly a gentleman, unfortunately has no income. Before her death, Glencora knew (and approved) of her daughter's attachment; the Duke, however, does not know of it, and is not at all likely to approve. Mrs. Finn (the former Marie Goesler), who was Glencora's closest friend, learns from Mary of her love for Tregear, and is faced with the question of either keeping silent, thus breaking faith with the Duke (who has entrusted Lady Mary to her care) or telling the Duke, and breaking faith with Mary herself.