Akuyaku Reijou no Kyouji

For years, Wellmy Ernest has made her half-sister Iora's life a living nightmare. She forced Iora to obscure her beauty, treated her like a servant, and even stole her fiancé. Their own father and his second wife joined in on the torment, cruelly confiscating Iora's belongings and forcing her to live in a cold, drab detached building. Now that Iora has turned eighteen, Wellmy is ready for the final insult: marrying her off to Aides Ormirage, the handsome yet heartless mage lord.
Tonight, at last, her brilliant scheme will reach its climax at the banquet held by Aides himself. Yet as the villainess prepares for her final act, she is acutely aware that all that awaits her is her own doom.
(Source: J-Novel Club)