You did it! We figured this one out! It is a truth universally acknowledged (by me for certain) that the Bitchery pretty much knows everything, and really, it's true. Scroll down to see the solution for this HaBO - and many thanks!

This HaBO comes from Rachel, who is searching for a recent contemporary romance:
I want to find a book I read fairly recently (in the last 2 years I think) that has gone missing. I don’t remember if I read it on my kindle or it was a paperback. It is a contemporary set (I think) in Texas.
The female lead is hired by the man to coach his sister on etiquette, and she moves in because it is a secret arrangement. She really wants him to hire her for business (he has international business interests and she is an expert on business etiquette or something – I think she speaks Japanese). Somehow their arrangement is exposed. The hero thinks she was using him for publicity/career growth so they fall apart for a bit.
I don’t know why I can remember so much about the plot and absolutely none of the names or author information. Help!
A possible Japanese-speaking heroine and an international businessman hero. I sense this is someone’s catnip!

This sort of sounds like a Harlequin Presents I read a few years ago: The Millionaire’s Misbehaving Mistress by Kimberly Lang.
I think Erin has it … from the Amazon description: As Dallas’s most eligible bachelor and heir to his family’s fortune, billionaire Will Harrison knows how to handle the paparazzi—but his little sister Evie is a worry….
Miss Behavior, etiquette expert Gwen Sawyer, has only three weeks to work her magic on Evie before a society ball, and so moves into Will’s luxurious penthouse. However, she discovers too late that etiquette is the last thing on devilishly handsome Will’s mind….
Yes, that’s it! Thanks!