that's literally all there is to it, inferiority and jealousy. she also mentions that erin is lovely, kind, rich, noble, and on top of it, born with the talent to become a swordmaster. she's the opposite. that's all there is to it, and such shallow things can make someone totally horriboe
It's because the stepmom is prejudiced thinking that just because someone is born as an innate talented genius, that means they won't have to work hard in order to get to the top (like she herself had to do in order to get there, and only to achieve the remarkable results of a "normal" level warrior, at best, and not those of a "once-in-a-lifetime sword master's), so she thinks the mc is unfairly privileged. But that is only her prejudice (being a born genius doesn't instantly mean that person will never ever work hard or that they will never have to strive hard to achieve their aspirations... moreover, sometimes talent comes with its own hardships)
i’m so confused why the stepmom hates her so much. like ok she’s a spy under emperor orders, the emperor wants to control sword masters but can’t, so he sends her to marry a respected advanced knight?? and the end goal is to kill the whole family? for what? and it seems like she’s pretty accomplished herself as she works for the emperor, so why is she so upset about erin having the traits of a swordmaster? i’d get it if her whole backstory was being jealous and feeling inferior to sword masters but it didn’t seem like a huge part of her shtick. idk if it’s the shit mtl or just the story but the motivations behind this whole setup don’t make sense to me.