Balthazar: The Spare by S.M. LaViolette

The Hale family is coming to London, and the ton has never seen anything like it…
Meet the Hales, a wild, wealthy, and witty family of loveable misfits who are about to take London by storm. If you loved Mary Balogh’s Bedwins, Julia Quinn’s Bridgertons, or Loretta Chase’s Carsingtons, you’ll adore the Hales!

Victoria Dryden gave Balthazar Hale the last thing he ever expected. A reason to stay…
Balthazar and his siblings are living the good life in the utopian community of Canoga, New York, where they have a degree of sexual and intellectual freedom few men—and even fewer women—have ever known.
But sadly, all good things must come to an end.
When their father dies, the Hale siblings discover two disheartening truths. First, they have an older brother none of them knew existed. Second, this stranger is not only the heir to an English dukedom, he’s also their new legal guardian.

Before the Hales know it, all five siblings are dragged to England to take their places in strait-laced, stuffy ton society, where their guardian expects them to make respectable marriages.
All Balthazar wants is a way out.
Until he meets her.