I come across the fanbox version first, where the sister got gangbanged, it's only later I found there was an official version. The one I read had dubious translation, the only thing I got from it is the sister being easily manipulated into sex, despite being visibly awkward around the guys, and ended up enjoying it. The brother got briefly mentioned.
When I started reading the chapters here, the title stood out: "The day I decided to make my cheeky gyaru sister understand in my own way". And then I read Day 0, the title immediately sounded ominous. We had the sister being incredibly rude toward the brother. The brother himself described her as "shitty gal", so this read like he looked down on her. He then said "You're gonna pay for this someday", I can only read this a threat and I linked the NSFW chapter to this, thinking to my self, "did the brother hired to scumbags to fuck/rape her?". And the final bubble says "I decided to gradually make her 'understand' in my own way", from what I know so far, this seemed like the brother is going to fuck up sister's life. At least, this is what the impression I got.
Next chapter, the SFW Day 1, it went the same way as the NSFW ver until the fade out, there was no sex scene but the sister returned somehow back at home, and collapsed on the entry way. We got a glimpse of the brother, and his eyes was drawn in a different way from the sister's, it looked menacing (eyes with vertical lines all over). Considering Day 0, I feel more convinced the brother is a terrible person out to destroy his sister. But next page his eyes were drawn normally, the brother even give her a bottle water, showing he "cared" about her. But to me, so far, he is just putting a mask, like a manipulative guy. At any rate, the chapter ends with the girl seemingly touched by the gesture.
Chapter 3, "Day 2" so I assume it continued just after the sister got back home drunk. Sister got sick and is bedridden. We found the reason the sister being rebellious, she found out she's adopted and I assume she felt alienated from her own family. Since she wasn't blood related, there was no reason for the brother to care for her. But the brother disagreed and affirmed they are family. In fact he even brought some gruel their mother prepared for the sick daughter. The thing that is interesting, is once again the brother's eyes were drawn in a menacing way. In p5, when the brother made to move away, the sister grabbed on his tshirt, and the brother just looked at her silently. Next panel, the girl looked worried/scared, the panel is darkening, as if something bad is going to happen, and she let go the tshirt. The opposite happened, the brother was glad his sister accepted to eat the food, and even joked.
Chapter 4, the title is now "The Day My Gal Sister Stepped Out". No idea when is this, at least few days after last chapter, enough time so the girl got healed. We've got the POV of a scumbag dude, he instantly made a sexual innuendo of being backed up since it's been for some time the girl last met with him, basically calling her a cumdumpster. He even described her as weak to pressuring in his thoughts, and I assume he was mocking her with "What a poor girl", since he had no problem exploiting her. This gives the feeling that the girl had already many sexual encounter with guys, Day 1 fanbox isn't the girl's first rodeo, there was no mention of hymen broken or blood. Next pages affirmed the idea, she's on her knees while the guy was standing over her, she got some fluid around her mouth. She got a message from her brother while the scumbag take a piss (conveniently). She got emotional over texting her brother, perhaps realizing she didn't like the way she lived her life lately, she regretted being so loose with herself. That's when she decided to stop, and she denied the scumbag's plan for another gangbang, and she returned home.
Before next chapter got released, I read a better translation of NSFW Day 1. One thing that wasn't clear before, but the sister's friend actually deceived her to come to a party with the scumbags. What a crappy "friend".
Another NSFW got released, during chapter 2 it seems. She dreamed of her brother molesting her and she got aroused by it. She's not beating the slut allegation for sure.
Chapter 5, "Making my cheeky gal sister understand christmas", some time had passed from last chapter. The siblings got along now, the sister stopped rejected her adopted family, she even got worried about her mother overworking. She felt down wearing the Santa cosplay, due to her past behavior I assume. The brother just called her cute and she got a tsundere moment. The brother's eyes were still scratchy drawn, but it doesn't have the ominous feeling from before though.
There's an untranslated NSFW chapter of the brother I think, where he is molesting his sister in her sleep. The brother got the typical Hprotagonist look.
Anyhow, that's how I read this whole thing so far. Sister found she's adopted, she decided to rebel, meet people and have sex, regret doing since her family keeps being supportive, and tries to be "normal" again. And she may or may not have unholy thought about her brother.