Akuyaku Reijou wa Nightingale wo Mezasu (Light Novel)

When Leysritt - a former registered nurse - awoke, she realized she had been reborn as the villianess of a novel. Because this world has such a limited knowledge of medical treatment, it's great that she met the prince and saved his life... But the prince's younger brother ended up taking a liking to her and became her fiancee! Wait... Did that happen in the original novel? Because the country is fighting a war, many lives will be lost if she doesn't constantly strive to improve her medical skills -- starting with the life of her fiancee! So now that her memories of the previous world have come back to her, she'll use that knowledge to save everyone!