Shion absolutely suffered severe psychological abuse. Even if she wasn’t Shion originally, she eventually took that identity and everything that came with it, including growing up hearing that “she” should’ve been murdered at birth by those who were supposed to love and care for her, and was treated as as much of a nuisance as that would’ve suggested she was to them for the rest of her life. Mion “took” the role that was originally hers with nobody so much as realising - and I think this is an important point, because there was nothing “different” about the twins in so far as their worth - and yet was brought up in an atmosphere of prestige, treated with such importance, whilst Shion was utterly, literally tossed by the wayside. She was sent to a school she hated, essentially forgotten about, and eventually forced through literal torture whilst everyone who again was supposed to love and care for her only looked on emotionlessly, including Mion, even though we know this facade was only her own way of dealing with the trauma. The environment the twins grew up in was a horrific one, but they’re both so stoic in their own ways I think it’s easy to forget.