I'm pretty curious as to why most of this group has multiple grievances with your sister, and what those grievances are. I feel like that could be really important.

Because on one hand, if it's relatively minor, then this sounds cruel and you should tell every last one who pitched in this to write down their problems and forcibly insert them in the anal cavity.

On the other hand, if your "bad tempered" sister has been repeatedly doing things that any normal person would understand are not okay, then they're not so far off base here (though Jane O. should still not have spoken for everyone without getting their permission, but that's neither here nor there). And the group chat where they vented was obviously not meant to be seen by you...and again, I'd be interested to know why so many girls felt they needed to vent about your sister in the first place.

Obviously she's your sister no matter what, but if objectively, the friend group has a point, then support your sister making life changes so that it doesn't happen again. If they're full of hot air, then support her finding new friends who aren't shit.

For what it's worth I lost a whole friend group before and I know how hard it sucks. I made a mistake and it prompted a terrible cascade of rumour and side taking and most of it happened behind my back so I know that sting of wishing people had just talked to me, heard me out. But I eventually took responsibility for my choices in that, mentally gave the burden of their choices back to them (instead of feeling like it was all my fault), and then moved on. Made better friends. Friends who'll hold me accountable for my actions...and who expect me to do the same for them. It's much healthier. I hope you and your sister both find that.