Honestly, the twins are pretty much the same to me. I feel sorry for Tsubaki being treated like shit, but I also understand Hiiragi's irritation and anger towards her.

Tsubaki:
- a can't-think-for-herself type of person,
- blind follower of her sister and felt betrayed when her sister didn't tell her that she was going to a different school when she's not really obligated to inform her about it
- threading on thin line between admiration and obsessiveness
- only joined hiiragi's interests to try to be better than her and come out on top so she can feel like they are equals

Hiiragi:
- is suffocated by her sister's "admiration" so she decided treating her like shit is the best move to make her stay away
- hates that her sister talks negatively about herself which makes her think negatively about herself too
- hates that her sister is always better than her even in things she doesn't care about which makes her feel insecure

Basically, they're both handling it wrong. If they stayed in that relationship, it would have been a lot more damaging, so I hope they realize how flawed they were from the start and choose to start over instead.