Dreamland by Sarah Dessen (2000)

"Perfect" older sister bound for her first semester at Yale runs away leaving behind 16 year-old Caitlin to deal with their parents and to make a new life without her. The sisters have always been close; Caitlin struggles to re-identify herself. Caitlin finds love with an unlikely and troubled boy; the relationship turns physically abusive and before Caitlin can figure a way out, things spiral out of control. Well-written, very realistic. Age: 16+