Shiawase ni Kurashitemasu node Houte oite Kudasai!

How did I become the beloved Crown Princess?! A Cinderella romance about an unfortunate young lady's dramatic reversal.

Mariabelle, the daughter of an earl, has her fiancé stolen by her greedy younger sister, and is then kicked out of her home for a crime she did not commit. Mariabelle decides to live as a commoner, but what awaits her is a sudden proposal of marriage from Crown Prince Riley!? Come to think of it, I attended the party to select the Crown Princess a few days ago, but...I never thought that I, the unloved older sister, would be chosen. To Mariabelle's confusion, Riley says - "I told you I wanted Mariabelle." The Crown Prince's overwhelming doting can't be contained!

(Source: Gaugau Monster, translated)