Runaway Road by Sue Divin

Ezra is 16. He's no stranger to crossing the line, but this time it's really bad - and there's no going back. His only hope of escape is to cross another line, the Northern Irish border, but that will mean leaving behind the only thing that really matters, his little sister, Evie. Evie is 9 and sees it differently. Ezra took her into care, into safety, the last time they ran away. Why shouldn't she go on the run with him now?