Smitten By the Spinster by Cassidy Cayman

When modern day actress Lizzie Burnet gets tossed back to eighteenth century London, she has to find a way to survive until she’s rescued by an unreliable time traveling earl.
Being a prim and proper chaperone to rich young ladies is just another role to her, and she finds a way to make a few shillings on the side by taking kickbacks from unscrupulous mothers who want their deadbeat sons to marry an heiress. She’s passing the time until she can get home. None of it’s real, and she doesn’t get attached.
Until a big handsome Highlander comes along and ruins everything.

Quinn Ferguson has to take his half sister Catriona to London to meet her English relatives for the first time. In order to inherit her vast fortune, Catie has to be properly married. Not entirely sure what that entails, he hires Catie a chaperone, and promises to be on his best behavior.
He can’t drink. He can’t swear. He can’t gamble. There’s to be no flirting and he has to somehow make himself seem less tall. Quinn doesn’t think the trip to England’s going to be very fun.
Until he meets the chaperone.