Nisani's Things

So, it's been a while since I read a manhwa where the fl reincarnated (?) into the northern Duke(?) or evil Duke (idk... I think she was the villainess). She was destined to die by the hands of the crown prince. One day when she was getting rid of the monsters in the lands, she finds the crown prince and then helps him to become the emperor. She was helping him to conceal his identity as the crown prince by magic(?) or dye...I don't remember. But, she introduced him as her cousin. Then her brother came back to the dukedom and started training the crown prince... that's all I remember. I really want to find this manhwa but I seem to have failed ಥ╭╮ಥ Please, if anyone knows the name of this manhwa...plzzz tell meeeee!!!!!
Btw, I also remember that the fl and ml met once when they were children and the ml thought that the fl was very tall and cool. Well, it was in the dukedom's blood to be tall or something...(╯°Д °)╯╧╧