Yeah, always feels like a cop out, either commit or don't but I feel like here it wasn't really the point since none of the characters are supposed to 'start off' as humans.
Like my usual objection is almost all the ones that do that are both fetishizing incest and trying to make it "more acceptable" by saying "they're not really siblings because they're not blood related", and that combination feels like (almost) the worst of both worlds to me. Like if the motive was just to say "they can safely have kids yay, my fictional pairing is safe from genetic defects!" I could at least
understand that, but that never feels like the vibe—always feels more like what it achieves is denigrating the validity of adopted family, giving the middle finger to the actual complexities of that situation, grinning and saying "it's barely legal, how immoral and exciting!", just yucky all around. (Token disclaimer: Everyone should be fairly free to like their own unhealthy genres of choice so long as they can understand and accept
why they're unhealthy)
But here there's seemingly quite different authorial goals in mind? She explicitly was even like "well okay I respect that we're not actually family in any sense of the word" first and that rings true to how we were shown they (weren't) really raised as siblings. I'd have to really nitpick to actually have anything to object to here.