It was not Sir Thomas who wished to end the secrecy of the rejected offer and he did not give in and tell the aunts of the proposal at Henry’s request. Henry forced the exposure because he freely talked about his proposal, Fanny’s rejection and his continued courtship and future plans with her at the parsonage:

In spite of his intended silence, Sir Thomas found himself once more obliged to mention the subject to his niece, to prepare her briefly for its being imparted to her aunts; a measure which he would still have avoided, if possible, but which became necessary from the totally opposite feelings of Mr. Crawford as to any secrecy of proceeding. He had no idea of concealment. It was all known at the Parsonage, where he loved to talk over the future with both his sisters, and it would be rather gratifying to him to have enlightened witnesses of the progress of his success. When Sir Thomas understood this, he felt the necessity of making his own wife and sister–in–law acquainted with the business without delay; though, on Fanny’s account, he almost dreaded the effect of the communication to Mrs. Norris as much as Fanny herself. He deprecated her mistaken but well–meaning zeal. Sir Thomas, indeed, was, by this time, not very far from classing Mrs. Norris as one of those well–meaning people who are always doing mistaken and very disagreeable things. (33)

Sir Thomas could either tell the aunts himself or allow them to discover it from those at the parsonage or gossip. I feel Sir Thomas handled it correctly. He altered Fanny before he informed the aunts so she was not blind-sided and secured a promise of silence from Mrs. Norris:

‘Mrs. Norris, however, relieved him. He pressed for the strictest forbearance and silence towards their niece; she not only promised, but did observe it. She only looked her increased ill–will.’ (33)