His Majesty's Hope by Susan Elia MacNeal

Now working as a spy for the Special Operations Executive, Maggie Hope's mission is to go to Nazi-occupied Berlin to deliver radio crystals to the resistance and then to bug a high-ranking Nazi's office. Maggie agrees to take the assignment, but when the Nazi is her mother, Clara (whom she thought had died when she was very young but recently learned that was still alive), the job becomes even more perilous. Interspersed with Maggie's story is that of her half-sister Elise (whom Maggie doesn't know exists) and that of her life as a nurse in Berlin. Elise doesn't share her mother's views, and when she finds out what really is happening with the children after they leave her care, she is propelled to action. This third novel of the Maggie Hope series continues the stories of Maggie and her friends, and immerses the reader in life in London and Berlin during World War II.