A certain age / Beatriz Williams.

Enhanced descriptions from Syndetics:

The bestselling author of A Hundred Summers brings the Roaring Twenties brilliantly to life in this enchanting and compulsively readable tale of intrigue, romance, and scandal in New York Society, brimming with lush atmosphere, striking characters, and irresistible charm.

As the freedom of the Jazz Age transforms New York City, the iridescent Mrs. Theresa Marshall of Fifth Avenue and Southampton, Long Island, has done the unthinkable: she's fallen in love with her young paramour, Captain Octavian Rofrano, a handsome aviator and hero of the Great War. An intense and deeply honorable man, Octavian is devoted to the beautiful socialite of a certain age and wants to marry her. While times are changing and she does adore the Boy, divorce for a woman of Theresa's wealth and social standing is out of the question, and there is no need; she has an understanding with Sylvo, her generous and well-respected philanderer husband.

But their relationship subtly shifts when her bachelor brother, Ox, decides to tie the knot with the sweet younger daughter of a newly wealthy inventor. Engaging a longstanding family tradition, Theresa enlists the Boy to act as her brother's cavalier, presenting the family's diamond rose ring to Ox's intended, Miss Sophie Fortescue--and to check into the background of the little-known Fortescue family. When Octavian meets Sophie, he falls under the spell of the pretty ingénue, even as he uncovers a shocking family secret. As the love triangle of Theresa, Octavian, and Sophie progresses, it transforms into a saga of divided loyalties, dangerous revelations, and surprising twists that will lead to a shocking transgression . . . and eventually force Theresa to make a bittersweet choice.

Full of the glamour, wit and delicious twists that are the hallmarks of Beatriz Williams' fiction.