Anyways as someone with many siblings I don’t understand how y’all romanticize incest so much. Yes I know it’s fictional. The only incest I’ll allow is step siblings that just met each other.
From my point of view and why I read incest stories, it's a level of drama
Note: I not only have siblings, but work in a field helping incest victims recover, I know very well the very real consequences of IRL incest
But we're talking fiction, and especially Yuri
Back in the "but we're both girls" days, being a lesbian couple was enough drama to drive a story
Now, fortunately, homosexual relationship are more accepted, more talked about, more "normal", and so "but we're both girls" isn't a steong enough drama to drive a story, at least not the kind of story needing drama to be told
So we had some new drama adding to the lesbians themes: actual discrimination, adult topics, be it work related, marriage related, sex related, and many other things. It can also be cheating, manipulation, NTR.
It can be anything and to be honest, I don't like to read most of those things.
Enter incest. Is those stories incest is the drama, so you don't need "more" to drive your story
It's back to those classic forbidden love tropes, we just replaced "but we're both girls" by "but we're sisters"
And that is often way milder than most of the driving dramas we have today
(There is also a history lesson to be made about the links between Yuri and incest, about the "sisters" relationship in Class S stories and such, but I'm not doing that today)