The taciturn fiancé is apparently actually a chatterbox who also happens to be in love with my younger sister.

Suddenly, Annemarie, Erna's younger sister, arrived at the mansion prepared for the newlywed life and began claiming that Hartwin, Erna's fiancé, was in love with her and that he was actually quite talkative. When Erna looked at Hartwin beside her upon hearing these words, he appeared very troubled and showed no sign of knowing anything about it. And the more Erna listened to the story, the more absurd it became, leaving her exasperated. There was a special reason why Hartwin did not do—or rather, could not do—the things Annemarie claimed he did.