Idk I have a theory that since she keeps inviting her brother along and seems to have an inkling of what's going on--that she's actually trying to bring the two guys together, but since they are both in denial she's kind of forceful and sneaky about it. She's also taken care of her brother her whole life, paid for his school too so I think she's trying to take care of his romantic life and try to push him into admitting his feelings. It seems like the author wants you to think she's sinister but I think there is fujoushi fire behind her eyes lol (๑•ㅂ•)و✧
I haven't read it but I totally get you--I'm not into disturbing and traumatizing plots. I don't even know how I got through season 1 of Killing Stalking at all or why I read it (I just wanted to know if the mc becomes the serial killer or dies in the end) but I will def stay away from an Uncomfortable Truth! Thanks for the heads up!
It's actually insanely good, it's just that it's so investing it really fucks with your emotions and has likely some of the most gut-wrenching scenes I've seen in a manga. It's also pretty similar to KS minus the sexualization but definitely in keeping with the violence and psychological aspects in some ways to a greater extent. It's kind of like a slightly less explicit KS with more in-depth character development and Save Me is also similar but with an even lower level of violence and sex but much more hateable villains and likable protagonists. An Uncomfortable Truth is probably the best of the three imo but if you can't take a good heart crushing I'd definitely recommend Save Me instead.
Happy to help, but pro tip, I'd read Uncomfortable Truth first, then Save Me as a bit of a pick-me-up afterwards and you can just pretend that it's the much nicer alternate ending to An Uncomfortable Truth, at least that's what I did in a meager attempt to heal the massive hole it left in my soul haha.

I mean, I've heard of sibling rivalries, but damn bruh, she scary. Dude should only be sleeping next to her with a helmet and a kevlar vest haha. Σ(  ̄□ ̄||)