In fact, it could almost be a stand-alone novel about characters with whom you're not familiar, because with the exception of Louisa Clark, most of the characters are new. There are vignettes featuring her sister, nephew and parents, as well as some involvement from Will's parents, but mostly this is about a woman struggling to move on after having lost someone she loves. It's about a woman who is caught between the theory of high expectations and the facts of mundane life. It's about being stuck, and it's about finding solutions in unexpected places.
The book does contain some Will-related surprises, and in that sense it is a sequel; but you could probably read it without having read the first and get the significance of 95 percent of it without trying.
The main thing to know is that if you expect to be moved or swept up in the exact same way you were by Me Before You, you may find this book a disappointment; but if you loved the character of Louisa Clark and want to know what happened to her, you will enjoy the journey. And there is a subtlety and a quality of realness to this book not shared by the first. The first one was the fairy tale; this one is the thump back down to earth and what happens next. They're really different books, but each has its appeal.