I always find it kind of depressing when I read oneshot anthologies because I end up finding tons of oneshots that have WAY better and original concept ideas than the 20th "I was reincarnated as an otome vilainess after getting hit by a truck" serialized manga I've read this week.
I'm frankly glad this is short—because at this length I can pretend the whole weird implied incest angle isn't a thing, and they're just BFFs forever (which I feel doesn't really detract from the nice parts of this, I might add)
Er... if all you care about is blood relations, than yes. A lot of us care about more than just that (and "incest" is still the word for those objections).
Even, say, the legal bits around grown-adult adoptive siblings marrying tend to be a bit tricky (perhaps not in Japan, I don't know).
Beyond any subjective yuck factor, in particular some of the non-health-related problems of incest are still a problem in this scenario (for instance, the inability to pack up your bags and leave if, say, the relationship turns sour).