Not really, that just sounds like a weak excuse. Exact same thing happened earlier in the morning with exact same outcome. Zero reason to expect anything less than spanking. She just framed him again, knowing what's gonna happen, because she's actually mean manipulative bitch who bullies someone else because she was bullied herself. At least that was my impression. I get that in that universe things like that are nothing extraordinary, its just after all the buildup with mutual help and trust we're back to square one again...
I just feel like that scene could be reserved for route where you refuse to help her. Would make sister character much more likable if she didn't betray all the trust. On the other hand if you skip those, you also skip jar, and neighbour shaming and pretty good chunk of content from that day. Still could have been written differently i guess, without whole framing. E.g. mom finds him in Iris' room and proceeds to punish him because you shoudln't barge in girl's room, or something like that.