The fact that they're step-sisters is relevant for only like 2 chapters. They still end up trying to hide it, only for when it's finally revealed basically nothing happens.
I liked that Haramin was a genuine friend of Yuzu. Everyone else just made gave me anxiety or annoyance that some new drama is being manufactured. Maybe sometimes I'm just a pussy for harsh realities, but I think most of it is the artificial soap-y stuff.
A lot of you guys seem to hate on Mei for being passive and toying with Yuzu's feelings for so long, but I didn't find her that annoying. There were many times Yuzu didn't fully reciprocate Mei's feelings and actions, so I found moreso that whole dynamic in general annoying, but not Mei specifically. I also think that's just who she is, someone who struggles to say no and is constantly trying to live up to others expectations, someone who is afraid of change. So even when she turns Yuzu down, and it delays their relationship, it still feels relevant to the progression of their romance and the plot.
I also don't think it's unreasonable that she struggled to Yuzu the truth, though maybe that's just because I personally am that kind of person. But she doesn't do any of it out of malice; if anything it's the opposite, because she's afraid of hurting Yuzu, even if in the long run she knows that she has to. I mean, if she did tell her the truth, what would happen? Either they'd continue dating with that looming over their heads, or they'd stop dating (while still being step-sisters and probably living together). (Of course, hindsight is 20/20.)
(When that plot was revealed I remember thinking, there's no way Yuzu hasn't thought or considered this in the entire time she's spent at this elite academy. It's mentioned in the beginning, and then apparently it never ever becomes a topic of conversation, nor has she ever just randomly ended up thinking about it. As the reader, it never crossed my mind either I suppose, but she's actually exists in that universe. Also, out of all the people in the world, Mei's fiance is just the manager of the cafe.)