The Stolen Bride by Jo Beverley
When Beth Hawley journeys to Lord Wraybourne's castle to help young Lady Sophie Kyle prepare for her wedding, she has to share her carriage with two unexpected travelers, the rakish Sir Marius Fletcher, who is stranded after his curricle overturns, and an unidentified elderly woman, found unconscious in a wrecked carriage.
As the date of Sophie's wedding draws near, Beth realizes some mysterious shadow from the past seems to haunt the bride and groom. The elderly woman, suffering from memory loss, may hold the key. Beth turns to Sir Marius for help, but her efforts are thwarted at every turn as she encounters a most unlikely villain-while her only ally seems to have his mind set more on romance than the danger at hand...