The Book of Thorns by Hester Fox

In the midst of the Napoleonic Wars and the rise of the floriography craze in Europe, two sisters separated at birth are bound together by a secret language of flowers passed down to them by the mother they never knew.

When Cornelia leaves her cruel uncle's home to join Napoleon's army as a traveling naturalist, her ability to heal any wound and bring soldiers back from the brink of death earns her praise - and exposes her to those who would exploit her powers for themselves.
Meanwhile, Lijsbeth lives in indentured servitude, her only respite her time spent flower arranging. When she meets a young English soldier and falls in love, Lisjbeth must decide whether to flee the clashing of two great armies at Waterloo or risk everything by staying. As the English and French armies collide in Waterloo, the sisters finally cross paths on opposite sides of the war.
With the sisters reunited on the battlefield, they must work together to solve the mystery of their mother's death, while surviving the war raging around them.