If it's not The Kneebone Boy (and it'd be helpful to us as a community if you commented whether you think it is or is not), it could be Homecoming by Cynthia Voight. There are four siblings in this book: James, Sammy, Dicey, and a little sister whose name I can't remember... But their monther leaves them in the car in the parking lot of a mall and they have to survive without her.