The romantic vibe is strong with this one. My predictions for the possible love interest of the two sisters are:
Sister number one: 1) Her husband, who is lost after having gone to war and no one knows whether he is alive or not (yeah, I know. He definitely is a love interest of her). 2) The German soldier, Hauptmann Beck, who has to stay at the house of the two sisters (please note, this guy is married as well).
Sister number two: 1) The stranger with a questionable past, who has helped her fleeing out of Paris. 2) The leader of the resistance in the small town the sisters are living in.
I just hope that this book is not going to be too sappy.