For what it's worth, I can tell you I did not add the tag. As for why I brought it up in the first place, it's because of the developments in chapter 5 (maybe I read it wrong, but it sure as hell seemed like the childhood friend was in love with the little sister) (then again, it does feel pretty weird she fell in love so suddenly, so maybe the author was throwing a curveball, so who knows?). Then, there's the translator's comment revealed in the credit pages of chapter six (though it might've been a joke). After seeing all of that, I checked MangaUpdates and, lo and behold, there was a shoujo ai tag. Now, I could be wrong, sure, but so far I've seen three indications of it being shoujo ai (the ones I mentioned above) hence why I wrote my comment. Now, to begin with, I'm not hating against shoujo ai. Some of my favorite stories are shoujo ai/yuri. However, I like to know beforehand that what I'm reading IS such a story, instead of being suddenly hit with it midway. (However, if this one does turn out to really be shoujo ai, I'll probably drop it, as I find the childhood friend insufferable.)
Anyhow, I apologize for any trouble I may have caused (and for my admittedly early, yet reasonable (imo), judgement). I guess we'll have to wait for chapter 7 in order to find out what it really is.
EDIT: also, on an unrelated note, I completely agree. There's definitely no NTR, as none of the characters are in a relationship in the first place. Absolutely no reason to add a NTR tag (and it's not as if there's one in the first place, lol).
EDIT 2: On that note, what earns a story a shoujo ai tag? Does a romantic subplot parallel to the main story between two women suffice or must it be the main focus of the story in order to be tagged as such? Because it may be we just have different opinions of what should be tagged as shoujo ai and what shouldn't (and I could be wrong, as I don't have any experience in this sort of thing).