The Six Days, by Anna Carolyn McCormally

But she understood. Nia had been raised by her eccentric grandmother and had experience warding off questions about her own parents, who had left her and moved to Europe when she was very young. Nia had never known them and she was nearly as touchy about it as Jamie was about his own missing mother. Maybe having this in common was what had drawn Nia and Jamie together in the first place; they had met in school and taken to each other immediately, like two broken things that together almost made a whole.