okay, I've seen the bad reviews for this drama but in my opinion, it needs some redemption... though I might get negative reactions with this review. But honestly, it's not a bad drama AT ALL! I've seen worse than this.
It's not so bad as reviewed by other watchers, I guess it's just a difference in preferences.

on their acting - I see most as describing their acting as bland and expressionless, but I think it's just that their characters have went through a lot to remain lively. Personally, I don't think they are "expressionless", and are just portraying how a "commoner/non celebrity" would react on a daily basis, cause I myself don't talk animatedly. Maybe their characters' MBTI are both "I" so they are more serious than other dramas' characters. Also, maybe because their characters are supposed to be classy, that's why they should look prim and proper (I doubt that you could see lively expressions from the Queen and the Royal Family on their daily lives). Honestly, I hate very animated acting, it feels exaggerated and over the top, and I won't be able to connect to the drama much since those kind of acting feels fake on the onset. I don't know any non-celebrity people who have that kind of lively and animated expressions. And the acting does not have to be in their facial expressions, but should be felt through their eyes, and watching them for 16 episodes, I saw happiness, sadness, hurt and love through their eyes, which is why i cried with them (leads, and the supporting characters). So in my opinion, their acting was "real life".

on their fashion - I noticed that this drama was in close collaboration with Esteem (a highly commended fashion agency in South Korea which has most of the top models) so I doubt that this fashion drama "has no fashion sense" like what others said. Fashion doesn't need to be extravagant, and could be simple and classy, like Sono's design. I think the image of Sono was more the prim and proper, simple and dressed down ladies.

on the story - Okay I know the confusion from the title, but I think they were being poetical with it. Like we should be breaking up since it's the right thing to do, yet our hearts tell that we should still go ahead and be in love. So while in the process of "breaking up" we stay in love, meaning there's no actual breakup at all, cause we can't let go of each other. On how it's possible to fall in love with brothers, I don't think it's the first drama in history that went that road. East of Eden (SKorea) sort of had a brother story, Scarlet Heart (China and SKorea) had the same, Fox's Romance (China), Tayong Dalawa (Philippines), Sand Princess (Thailand), and I'm sure there's more. And the attraction started way before knowing the connection with the brother, so I'm assuming it is possible to happen in a DRAMA. That's what fiction can bring to the table, more of listening to the heart than the mind.