Senjou no Seijo: imouto no Kawari ni Koushaku Kishi ni Totsugu Koto ni Narimashita ga, Ima wa Shiawase desu

Unlike her younger sister, who is known as the Saint, Finne is rather unremarkable, and has been treated poorly at home. After being sent to a dangerous battlefield as a military doctor, Finne meets a critically injured duke knight named Leon and uses her healing magic to save him from the verge of death. Then, everything in her life starts to change dramatically…?!
(Source: K Manga)