Kago no Naka no Ryuu Seijo: Shiitagerareta Hakushaku Reijou wa, Kyou mo Dekiai sareteimasu

The Dragon Saintess, tasked with caring for the black dragon Gramvint, who protects the Kingdom of Elvasti, is the official fiancée of the kingdom's prince. The current Dragon Saintess, Ludia, has dedicated her life since childhood to taking care of the enormous dragon that resides in a giant cage. She was also engaged to the prince, Edik. However, one day, Edik suddenly summoned Ludia and informed her of her dismissal as the Dragon Saintess and the annulment of their engagement. To her shock, the new Dragon Saintess was her stepsister, Leila. What's more, Edik told her that her next destination would be marriage to the Black General, Wilfried. But Wilfried said "You and I are not a good match," and rejected the marriage...!? (Source: Shueisha, translated)