I think you're mixing up a lot of the different versions of the story. First version she was just a normal slut. Bro was trying to make her "understand" what "Good sex" is like.
Second version has her be an inconsistent mess who starts off shy and meek until the sex starts, where she then flips a switch and loves every second of it with the fervor of an unhinged sex addict. This is entirely consensual on her part. From this version on, bro isn't after her body. In fact, bro's sexual interest in her completely evaporates, and we only see him care for her as a sister.
The serialization has her not even know these people, and actually make an effort to remove herself from a bad situation.
As for the FB, I took it to be a "I'm not happy about the being blackmailed into this, but also I'm literally incapable of not having an orgasm after you've gone well out of your way to make sure I do"-type thing.
I'm not mixing up the various versions any more than Egaki herself does. She says elements of the original story aren't canon due to Pixiv stepping in or well-deserved audience backlash, but all of the original content before the serialized version doesn't really contradict each other, and these allegedly separate versions even reference apparently non-canon events from what's supposed to be a "different version" regardless. I think it's pretty obvious Egaki was just hiding her intentions and expecting the audience to read between the lines. The serialized version is the only one that's truly distinct, and that's only because all actual erotic events have been clearly excised out of professional necessity.
I understand the details are
supposed to vary between them, but what I'm getting at is that, either way, the genie is already out of the bottle for this disjointed train wreck of a series. As you yourself said, "[Egaki is] making the series she actually wants to make in the FB. She even said that that's where she would be "true to herself"." I haven't looked into her exact words, so I'd appreciate it if you could point me in the direction of the post she said it, but I'm not inclined to be skeptical. We know what the initial concept was supposed to be, we know what Egaki herself prefers, and we know what's simply her capitulating to audience pressure or needing to censor the idea because creative expression is getting smothered to death on Pixiv.
So she's basically doing this sanitized serialized version just in hopes of getting a paycheck. At best, it maybe represents more of what the original concept was supposed to be where the brother was a predatory yandere who came off as wanting to monopolize her before Egaki had to nix that because of Pixiv rules, but since it has no erotic content, we can't be sure if it's taking more after the (seeming) later direction where the brother's intentions are wholesome. I'm not convinced of anything myself; he's still drawn as a creepy guy with no highlights in his eyes, so he could just be using the idea of a pure sibling relationship as a manipulation tactic, but again, the concept is completely muddled by now, so who even knows what this story is supposed to be about?
Meanwhile, Egaki still shits out the NTR garbage that's apparently "true to herself" on her fanbox, defeating the point of the serialized direction. It doesn't matter if it's not strictly canon, because it's ultimately the same concept and represents an element of what the mangaka means to convey through the idea and its characters.
It'd be one thing if Egaki had abandoned the prior version entirely and we could say it's just an earlier draft or basis of the idea, ala Nagatoro's serialization, but it's not that. It's a companion piece, but usually content behind a paywall like this is used to cater to the audience by showing the sex the default version can't (ala Zyugoya's or Hamita1220's works) or an "alternate possibility" where erotic content happens when it wouldn't otherwise. In this case, you're paying extra for NTR because Egaki's initial incest plans went up in smoke, though, so ultimately the serialized version and its associated erotic content are at odds with each other. It's just a mess.
Having said all that, it's really not that big a deal. Ultimately it's just an artist stumbling over themselves trying to make money off of an idea they apparently care about, but ending up with nothing but poorly written smut.
wwww
