Formerly, the Fallen Daughter of the Duke

Formerly, the Fallen Daughter of the Duke is a reincarnation isekai.

Claire Martino is the engaged to the crown prince of Noston Kingdom. However, she finds everyone favoring her parental half sister Charlotte after her baptism for magic proved her superior. The night before the graduation party, Claire finds out that Charlotte was lying that Claire was tormenting her; everyone just bought it, without any proof. Seeing she needs to leave before she's doomed, Claire packs and heads to the next country; along the way she has a nap, which reveals her past life memory of being girl named Minami. To Claire's shock, she's in the otome game Eternal Love.

To Claire's great luck, she meets Prince Vic of the next kingdom while he's out with his knights. And visits her mother's birth land, where she unlocks her true magic potential. Things only get more complicated when it turns out her mother was the princess of a kingdom ruined by a neighboring nation, and was likely killed out of paranoia; meaning the same fate could await her.

Trivia

 * Charlotte's hair looks like devil horns, a subtle way of showing her true evil nature.
 * In fact, in the real world Minami was from, the players know she's a scheming, backstabbing wrench; which gives Claire a lot of pity.
 * Even without her memory waking up, Claire knew to cut her losses before she could be humiliated by her evil sister.
 * Charlotte noted that this messed up her plans, as she needs the prince to stupidly annul the engagement in public and announce Charlotte as Claire's replacement. Plus she intended to force Claire to do all her work.
 * Charlotte is an illegitimate child, explaining why she is so greedy for wealth and status. Even though she was given unconditional love from Claire, Charlotte sought to replace her as the next queen.