Moto Ansatsusha, Tensei shita Kizoku no Reijou ni Narimashita.

The story is about Serena, a former assassin who was reincarnated as a duke's daughter after failing her mission. She initially thought that pretending to be a noble would be easy, but she struggled with the gap in perception between herself and her mother, who had a flighty personality, and her adoptive sister, who deceived those around her in an attempt to push Serena out of the picture. Despite skillfully dodging the harassment she faced at the tea parties full of rivalry and navigating the complex social life of a school embroiled in love and hate, Serena's cold demeanor resulted in her being ostracized and isolated. One day, she discovers that her adoptive sister is engaged to the second prince, which leads her to run away from home. However, it is at this point that the first prince, Evan, appears in front of her... Follow Serena, a former assassin who knows nothing of love, as she charges forward into the aristocratic society of another world in this comedic, occasionally romantic tale. (Source: MangaDex)