Daniel Brühl performs well in this improbable but diverting drama. He portrays a man whose mother (Katrin Sass), a proud citizen recently honored for her socialist dedication, lives with him and his sister, “Ariane” (Maria Simon), in East Germany. However, it’s 1989, and the Honecker regime is visibly crumbling; protests are occurring, and among those involved is “Alex.” When he's arrested during a demonstration, she…