that’s exactly it! while i don't believe that Darcy pushes the "matchy matchy" outfits in the way Louise is claiming - sure, she may not complain about having to wear matching clothes as Pearl because she knows it's expected of her, but that doesn't mean she actually encourages it - but even if that
were the case, it's veyry clear that the matching outfits are very much Louise's choice. it wouldn't be so jarring and seem so weird if the girls wore matching pyjamas or similar leggings or whatever, and probably wouldn't be especially noticeable - it's the fact that Louise makes such a huge deal out of it - which give sthe impression that it's very much encouraged by
her, not Darcy and is likely another behaviour fuelled by her concerns about blended families/the girls being half sisters etc etc - and the dramatically over-the-top ruffled, frilly dresses with ribbons and bows that she insists that they wear. whether Darcy likes to match with her little sister or not, i don't believe that she chose to wear such babyish outfits - especially those matching sailor dresses
- or those hideous knitted mini-politian suits! as you said, the clothes Darcy dresses are entirely inappropriate for an eleven year old, regardless of whether they match with her little sister! Darcy isn't a dress-up doll for Louise to play with, and she's at an age where she should be allowed, and encouraged, to wear what
she likes, express her own personality and identity, and develop her own sense of style - not be expected to dress according to Louise's preferences, whether in ruffled toddler tutu dresses or weirdly formal attire, i which she looks painfully uncomfortable and embarrassed!