Moto Ansatsusha, Tensei shita Kizoku no Reijou ni Narimashita.

High Society Is Rough For Assassins! Enter Lady Selena, the former assassin who assumed it'd be easy to step into the role of pampered mademoiselle, but who finds herself aggravated by how differently she views things compared to her flower-brained mother and her scheming, deceitful (yet utterly ineffective) stepsister. She navigates tea parties filled with ladies attempting to assert their dominance, attends an academy thick with ulterior motives, and dodges harassment flung at her with an icy glare and fierce counters, but it's that stone-cold heart of hers that leaves her isolated and alone. One day, she learns her stepsister has become engaged to the second prince, a fact that forces her to run away from home. She doesn't get far before bumping into someone, however, and that someone just happens to be the first prince. Come watch as the cold-blooded former assassin who's never known love dives fearlessly into noble society! (Source: Cross Infinite World)