The Girl Behind the Wall by Mandy Robotham
When the Berlin Wall goes up, Karin is on the wrong side of the city. Overnight, she's trapped under Soviet rule in unforgiving East Berlin and seperated from her twin sister, Jutta. Karin and Jutta lead parallel lives for years, cut off by the wall. When Jutta finds a hidden way through the wall the twins are reunited. But the Stasi have eyes everywhere, and soon Karin is faced with a terrible decision.