I'm not big on spoilers usually, but since you asked... The ending ties up the main mystery of who Tobias Hawthorne really was and why he chose Avery. The biggest twist is that Toby, the 'dead' grandson, is actually alive. He'd been living as Grayson's friend, Jameson's rival, and the groundskeeper, Xander's buddy, this whole time. He faked his death because he believed his grandfather killed his mother. The will's real purpose was to bring him home and force the family to confront their past.
Avery doesn't get the money, not all of it anyway. She decides to split the inheritance with the four Hawthorne brothers, which I thought was a fitting choice for her character. She gets the Hawthorne House, but more importantly, she chooses Jameson. The last few pages have them racing off in his car for another game, which sets up the sequel perfectly. The whole 'alive grandson' thing felt a little rushed to me, but it does make you rethink every interaction Toby had with the brothers while he was in disguise.
Honestly, the part that stuck with me was Avery's final decision about the money. It showed she was never really in it for the wealth; she was solving the puzzle for the truth and for a place to belong. The book ends with more puzzles hinted at, obviously leading into 'The Hawthorne Legacy'.