This was a beautifully written story, where the attempts to solve the mystery of who killed Alison takes a backseat to character building. You learn so much about the characters -- the sister and the prime suspect especially -- that it makes it much deeper. There's more to these characters than just the murder mystery aspect. The sister was not particularly likable, but again that made her more real, seeing all of her and in particular how this event shaped her entire life. It was also very interesting to see the dynamics of the characters on this resort island and what life is like for someone who lives there vs. the perspective the tourist will see.
I'd highly recommend this book to anyone who likes character-driven novels. This may not be for you if you really want something focused on the whodunnit aspect, but if the mystery is less important than the characters, than you should love it.