I’m looking for a book. The book is about a girl who is a twin to the princess. She is the main character in this book. No one knows she exists and her only job in life is to protect her sister because her sister is going to be queen. Only a few workers around her castle know she exists.
Then they start to get threatened and to make a peace deal with another kingdom. The king says he will send over the princess, his daughter, as a way of showing that he will not harm their country and trusts them. Instead, they send over the twin, who had a cool name that I think started with an 'R.' The twin has to pretend to be her sister, even though she was trained in combat and not anything fancy, or manners like a princess should have.
She gets there and ends up falling in love with the prince from the other country thing and they run away together, because her dad, the king, is planning on bombing the other country.
That’s all I can remember. Also, there may be like multiple books to this series, but this is the first book I am looking for, so please help.