Zutaboro Reijou wa Ane no Moto Konyakusha ni Dekiai Sareru

Betrothed to My Sister's Ex

Marie, the second daughter of a poor baron’s household, is treated like a servant by her parents. Even at Marie’s birthday party, her beautiful elder sister, Anastasia, is the star. Outside, Marie bumps into Count Kyuros Granado, who falls for her at first sight. Yet, due to a misunderstanding, the Count proposes to Anastasia. After a tragic accident, Marie must marry the count instead!
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(Source: Crunchyroll)