Well this discussion sure is/was lively. There are a few points I wanted to delve into detail, given new available information. I believe what we have here is a classic switcheroo, where the author/artist has crafted a story that seems to follow a particular dark path, but the actual story takes place in a serialized release, whereas this version only shows the most controversial highlights. There are some hints that the really bad stuff didn't take place at all in this version, even though it could easily be assumed that they did:
Firstly, big sis is only hanging around the dirtbags because she feels isolated and lonely, being the only member of her family who is not blood-related to the others. She has a foul mouth, which would drive away any 'normal people' in school from wanting to become friends with her. She's also really pretty, so naturally the worst elements say and do what they can to coerce her into bad situations. One of the guys even mentions that it was hard to get her to actually come to their place, and even then, it was done under a false pretense because her (only?) friend had lied to her about the number of men to be present. She's easily pressured into drinking to unconsciousness, but there is no inward monologue which would indicate that she's into the prospect of being "sexually used" by all these guys. There are plenty of gal/gyaru type women in manga who look the part, but don't actually play the part; those who fully embrace this lifestyle tend to be very obvious about it--like big sis' friend.
Secondly, even if the dudebros even knew where she lived, it's unlikely that they would have gone through the trouble of dropping her off INSIDE her house like that. I mean, would they want to risk getting caught? Her bag was left behind, because she was taken back home by someone else in enough of a hurry that it couldn't be grabbed as well. She assumes that she just came home on her own, but doesn't remember anything at all beyond the drinking.
Thirdly, when she wakes up, her clothing and overall appearance are in much less worse for wear than what she would look like if all those drinking guys had slobbered all over her in sex. Drunk sex is pretty clumsy. I suppose that her lying friend could have fixed her up, but that's too much of a stretch to happen with absolutely no indicators. Besides, she was already getting busy elsewhere with another one of those guys. Big sis doesn't monologue at all about anything actually happening to her body, and if she were raped by multiple people, there would be definite indicators on/in her.
Lastly, when big sis returns to the 'love shack' to recover her bag, the guy present mentions being "backed up because she wasn't coming around". However, it's only been like 1 day or so later, and she acts like she doesn't even know what the phrase means. Imo, the guy hadn't been 'getting any' because he had hoped to have sex with her during the prior encounter, but she was whisked away from the place before he could have his way with her. She's also resting on the ground on her knees because she's wearing a skirt, and wouldn't traditionally sit on a couch wearing those kinds of clothes, not because she had given the guy a blowjob. It's surprising that this guy didn't just go after big sis' orgy-compliant friend.
Now, I could be totally wrong about all this, but it just seems that there are enough indicators that the author/artist ultimately had another story path in mind, but created this version in a manner that would not only stir up controversy, but also make it look like the worst fears of what happened to big sis were confirmed if one didn't read between the lines. The author had also been publicly adding fuel to fire about this as well.
Just some thoughts, I'd like to hear someone else's opinion about this version of the story.
edit: Okaaay, so I just realized that there are multiple versions of this story and what I described above is the pre-serialization leading into the canon main story serialization, I think. It's a bit confusing.