Akuyaku Reijou no Yakuwari wa Oemashita

Yukino Amaishi signed a contract with God to save her sister. She was then reborn in a different world as Lefina Eilferia, the daughter of a duke and the Crown Prince's fiancée. After successfully completing the objective of giving the heroine and the prince a happy end, Lefina, who played the role of the villainess, ends up working in the castle as a maid. (Source: AlphaPolis, translated)