You know what, this is good. The 'openly promiscuous' girl senses something uncomfortable leaving him with another girl, much less her (half?) sister. She might be one of those people who don't learn shit until it actually punches her in the face. She's the one pushing MC getting to get a sex friend in the first place, let her feel the consequences of her actions, have her heart torn out from her and thrown to the ground.
And the sister/class rep knows what's going on, too, or maybe she doesn't, I don't care. Someone make that girl experience heartbreak and have MC pick up the pieces for her, because none of her friends don't look like they're gonna do anything to help her get better at this point.