We Are All Completely Beside Ourselves - Karen Joy Fowler
Meet the Cooke family. Our narrator is Rosemary Cooke. As a child, she never stopped talking; now that she's started college, she has wrapped herself in silence: the silence of intentional forgetting, of protective cover. Rosemary is now an only child, but she used to have a sister the same age as her, and an older brother. Both are now gone—vanished from her life. Her once lively mother is a shell of her former self, her clever and imperious father now a distant, brooding man. And there was something unique about Rosemary's sister, Fern.
You'll have to find out for yourself what it is that makes her unhappy family unlike any other.