Until Morning by Shannon Gilmore

They broke the rules for fun. They fell in love forever.

What does your best friend’s rake of a brother, a notorious gambler, and a winter masquerade have in common? The perfect storm for Breaking the Rules of the Beau Monde.
When Adeline Priestley receives an invitation for a Christmas masque from her friend Lady Clover she’s in for a treat because not only will her best friend Evelyn Markham be there, but she’ll be bringing her brother as a chaperone. She may have made a cake of herself with her girlish crush on Winn Markham, but the appreciation in his eyes at her come out had been more than complimentary. Three years later and she wondered if his warm, umber eyes still held the same spark.
Winn Markham hasn’t been home for three years, and just when he thinks his father will finally forgive him for his past dealings with card sharps, he’s thrown into babysitting his younger sister for a week of post-Christmas cheer. He’s not thrilled about the prospect of spending a holiday with his sister and her silly friends until he realizes that Adeline Priestley will be there.
No one: not Adeline, Evelyn, Clover, or Winn and his friends, would have ever guessed that the week would turn into such scandalous fun when one of the girls suggests a week of Breaking the Rules of the Beau Monde.