The Final Gambit by Jennifer Lynn Barnes

To inherit billions, all Avery Kylie Grambs has to do is survive a few more weeks living in Hawthorne House. The paparazzi are dogging her every step. Financial pressures are building. Danger is a fact of life. And the only thing getting Avery through it all is the Hawthorne brothers. Her life is intertwined with theirs. She knows their secrets and they know her.

But as the clock ticks down to the moment when Avery will become the richest teenager on the planet, trouble arrives in the form of a visitor who needs her help—and whose presence in Hawthorne House could change everything. It soon becomes clear that there is one last puzzle to solve, and Avery and the Hawthorne brothers are drawn into a dangerous game against an unknown and powerful player.