Yukina (cheating GF) has a strong inferiority complex against Yukito (BF being cheated on) since the latter is cool, responsible, and mature (which was because his mother died, his mother figure was his strict lawyer aunt). Yukina meets a college guy at her part-time job that is basically the antithesis of Yukito, sloppy and has to be taken care of, and she gets off at the idea of cheating on Yukito with a guy who is the complete opposite. Yukina has serviced the college guy with her hands, mouth, and chest so of course it's already unacceptable, but she justifies it in her mind that since she didn't go all the way, she can still be with Yukito. After Yukina gets dumped, she admits to herself that instead of feeling inferior she should have made an effort to be a better person to feel worthy of being with Yukito. She's a piece of sh*t yeah, but she's a well-written piece of shit. The author could tell me that Yukina is a real person and I'd believe him.

Yukina's younger sister, Setsuna, is absolutely bonkers. Like damn, woman, I know that you have the hots for Yukito since he helped you with your timidity and self-esteem issues and I agree that Yukina is a piece of shit, but you didn't have to do what you did. Maybe a hard scolding from their mother, losing her allowance, forced to dogeza and apologize to Yukito's dad and aunt, stuff like that sure, but what actually happened seemed like something that you'd read on the r/NuclearRevenge subreddit. Setsuna manipulated things so that Yukina would lose her place even at home, knowingly destroyed Yukina mentally and it caused Yukina to commit not alive (it's also implied that Yukina took the college guy with her to hell).