I FAKED AN ENGAGEMENT TO A DUKE'S SON BECAUSE MY SISTER STOLE MY FIANCÉ.

Anastasia's engagement is called off by her parents and the engagement party that was supposed to take place on her birthday is suddenly turned into an engagement party for her sister Laura and her fiancé Nikolai. Of course, she has to attend the party, too... Before she knows it, the school has found out about the engagement. Anastasia is humiliated and shocked, but Lewis, a junior student proposes a fake engagement between the two! Anastasia, no longer afraid of anything after experiencing a broken engagement and the pity of others, becomes desperate and plans to crash the engagement party with her fake engagement announcement. (Source: MangaPlaza) — Description from AniList community (anilist.co).