My Replacement Fiancée, the Saint's “Failure” of a Sister, Is Acting Strange

Oswald, the most powerful magician in the land, was well aware of his exceptional magical abilities and striking good looks. He understood that charm was also necessary to navigate life successfully, so he carefully cultivated a pleasant facade. This led to an engagement with the Saint, a union he believed would be a harmonious arrangement.
However, fate took a cruel turn when Oswald, while protecting the Saint during a battle against a magical beast, suffered a grievous wound to his face. Not only did a disfiguring scar mar his visage, but lingering miasma seeped into the wound, draining him of his magical power and leaving him bedridden.
Stripped of his strength and beauty, Oswald received a heartless notice of a broken engagement, along with a letter offering a replacement… the Saint’s “failure” of a younger sister.
Rumors whispered of the younger sister being mistreated by the Saint.
Resigned to abandoning his carefully crafted persona, Oswald accepted the new engagement, allowing his new fiancée the freedom to live as she pleased.
However, her behavior struck him as odd… Was she truly incompetent and ab*sed as the rumors suggested?