https://www.cmoa.jp/title/285005/

“You think you can sleep with me and brush it off as a mistake?” Estelle, the daughter of a viscount, suddenly finds herself reincarnated as a side character in a popular web novel. In the original story, Estelle’s role is to steal the heroine’s fiancé, the second prince, and be condemned and exiled for it. Determined to exit the narrative quickly, she plans to pretend to seduce the second prince, just as in the novel. But she accidentally approaches the wrong man, the protagonist, First Prince Oscar! A heart-melting fantasy romance between a hapless mob noblewoman and a possessive, perfectionist prince.