Jinx by Margaret Wild
This is a moving, mulitlayerd story written in poems. The poems are short, rarely longer than two pages, and each is told from one of the main chararcter's perspectives. The main characters are Jinx, her three best friends, and various adults who feel the pain of events in the book. One of these adults in Jinx's mother, who has had to deal with her husband leaving her when they had a Down's syndrome baby. That baby has grown into Grace, Jinx's beloved sister. Jinx calls her father "the Rat" and hates visting him and her stepmother.
Jinx didn't start out as Jinx. She used to be Jen, the good girl who always did what she should. Then her boyfriend, who she loved dearly, kills himself. She is heartbroken, but still Jen. After her next boyfriend dies in an accident, Jen starts calling herself Jinx and becomes more and more isolated from those around her.