Even though the sauna situation strained suspension of disbelief in and of itself, what got me was having to go back and re-read the previous chapter to make sure I wasn't getting their mothers mixed up. Seeing Shimamura's mom berating her so much in the previous chapter, I feel like Shimamura's mom is the one who deserves the dressing down.
The most surefire way to get your kid to internalize the idea that they're no good is to constantly tell them that they're no good. Between being distant and constant belittling, I'll take being distant any day of the week.