Meria was a commoner! She was a mercenary and the Duke was in love with her and she eventually said "ok, let's get married... She does come to love the Duke, but as she was a commoner and entered the high circles, people looked down to her, because of her past as a mercenary, she developed a inferiority complex ... When her kids born, especially seretina, she wanted her to be the perfect lady, to compensate for her lack of "nobility"... She slap her because she saw herself in seretina and her inferiority complex kicked in