Manga About A Girl Who Dresses Like A Guy In A Royal Family?

I can't remember the name. Its about this girl who has to take place of her dead brother to protect this princess(teenager). She is like some sort of bodyguard. She dresses like in a vest and black suit. Only men are allowed to take the position but she hides the fact that she's girl and is forced to join because its a tradition that runs in her family. The princess she is protecting is like a teenager who is arranged to marry this boy who is lazy and laid back but also some sort of prince.

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