The Unhoneymooners by Christina Lauren
In The Unhoneymooners, perpetually unlucky Olive Torres somehow manages to be one of the only two people at her twin sister Ami’s wedding who does not come down with violent food poisoning, the other being Ethan Thomas, brother of the groom and Olive’s personal nemesis. The honeymoon—an all-expenses-paid trip to Hawaii that Ami won in a contest—can’t be rescheduled, so the newlyweds urge Olive and Ethan to go in their place.