Well, this is a pleasant, there is the classic pure nice heroine and the bad dark villainess...on the surface!
Instead it's the manipulative selfish younger sister forcing the older sister to act as the villainess and can't survive another way as the younger sister had added to the web that snared the older sister.
And to add to this unexpected base for the story the male lead that would normally fall for the younger sister just dosent.
This guy is so far a straight forward guy, not simple, but straight forward. Kinda like I'd you had a kid in an adults body with the instincts of a dog. If they don't like you for some reason, a normal adult person wouldn't be able to figure it out, but a kid or a dog can.
So he sees the whole classic bulling seen unfold, all normal adults see the same thing, but he sees an 'act' a game, a play. Complimenting the older sister on all her efforts on her villainess role and says how poor of a actor the younger sister is.
Just another lovely change in the spices that make another good villainess and heroine story.
Here's rooting for the older sister and having a happy life with our straight forward male lead.