Akuyaku Reijou no Kyouji: Konyakusha wo Ubaitotte Gishi wo Oidashita Watashi wa, Douyara Ima kara Hametsu suru you desu.

Once a commoner, Wellmy Ernest ascended to nobility when her mother married an earl. She tormented her same-age half-sister at home and school, stole her heirship and fiancé, and finally sold her off to the allegedly cruel Lord of Magic. It's not like her plain, meek sister could oppose. Wellmy got everything she wanted. One day, Wellmy receives an invitation to a late-night soiree from the Lord of Magic, likely to announce her sister as his fiancée. Wellmy attends with her mother and father, but what awaits them isn't the brilliant banquet they were expecting...it's a tribunal condemning them for their abuse of Wellmy's sister. Which means everything has gone just as Wellmy planned... This is the story of a lone villainess who plans to make her sister happy even if it means she's hated in return. (Source: Square Enix)