They're cousins, not even necessarily first cousins, it's not very incest.
I mean, pseudo-incest is a tag mostly because there are people who get distressed over a piece of paper somewhere saying two strangers are now "sisters". Shared DNA certainly isn't going to be relevant in a schoolgirl yuri work, either. For the purposes of squick and prejudice, any reality on the ground is secondary to whether it can be labeled "incest".
"Pseudo-incest" can still be a real problem, not because of "piece of paper" but because of being in a family and possible power relations and difficulty disentangling. Like in Citrus, Yuzu and Mei met at 15? older? they may be stepsisters, but they're not really sisters the way adopted small kids could be. OTOH, they do share parents (well, parent) now, and a home (even a bedroom), and Mei is basically living with Yuzu's mom since her dad is AWOL... so if their romance blew up, things could get really awkward, and Mei would be a minor trapped with two potentially hostile relatives.
(Or not so trapped, given her rich grandfather, but work with me.)
Or for a similar case, https://dynasty-scans.com/chapters/a_love_smeared_in_ashes Again, very late adoption case, but the adoptive/half-sisters having a romance would be playing with fire if anything goes wrong. (Though I guess these actually share do share some paternal DNA.)
And of course if someone's adopted very young, then emotionally they basically are siblings, even without sharing DNA.
My point being, "pseudo-incest" can be about real issues, not just pearl-clutching. Though of course it can also be a way of getting incest thrills without it being 'real'.