Dying Day by Robert Ryan

In this Cold War spy thriller based on actual case files, a beautiful young woman is willing to do whatever it takes to bring her sister home
In the darkest days of World War II, Laura McGill and her sister, Diana, were willing to go behind enemy lines on behalf of Britain's Special Operations Executive. They knew the risks and were willing to take them for king and country. What they did not expect was that their government would forget all about them once the war was over.
It is the spring of 1948, four years since Diana disappeared inside occupied France, and Laura is sick of being ignored-so sick that she is willing to kidnap the head clerk of the SOE at gunpoint in order to learn the name of the spy who ran her sister's last mission. The name she gets, James Hadley Webb, will take Laura to Berlin, a divided city on the verge of being cut off from the Western world. Webb is on the front lines of a new kind of war, a shadow war of influence and intrigue-and losing. Laura's arrival, unwelcome as it is, may be just what Webb needs to turn things around-and stop his agents from dying.
Dying Day is the 3rd book in the Post-War Trilogy, which also includes After Midnight and Last Sunrise.