Task: Question Generation
Input (English):  The plot concerns the children of the Duke of Omnium, Plantagenet Palliser, and his late wife, Lady Glencora. When Lady Glencora dies unexpectedly, the Duke is left to deal with his grownup children, with whom he has a somewhat distant relationship. As the government in which he is Prime Minister has also fallen, the Duke is left bereft of both his beloved wife and his political position.
Before her death, Lady Glencora had imprudently given her secret blessing to her daughter Mary's courtship by a poor gentleman, Frank Tregear, a friend of Lord Silverbridge, the Duke's older son and heir. Mrs. Finn, Lady Glencora's dearest confidante, somewhat uneasily remains after the funeral as a companion and unofficial chaperone for Mary at the Duke's request. Once she becomes aware of the seriousness of the relationship between Mary and Frank, Mrs. Finn insists that the Duke be informed.
The Duke's two sons also prove burdensome. Lord Silverbridge follows the wishes of his father by entering Parliament. He had proposed to Lady Mabel Grex, whom he has known all his life. She turned him down, although with an indication of a more welcoming answer another time. However, Lord Silverbridge becomes enamoured with American heiress Isabel Boncassen. She agrees to marry him, but only if the Duke is willing to welcome her into the family. At first, the Duke disapproves; and he disapproves even more of his daughter's suitor. To add to his troubles, Gerald, the younger son, gets himself expelled from Cambridge after attending the Derby without permission.
However, by the end of the book, the Duke grows closer to all three of his children; he allows the engagements of both son and daughter, and he is invited once more to take a part in the government.
Output (English): Who had Lord Silverbridge proposed to but was turned down by?