Yeah the mom was cheating. Satoko punched the wall out of anger and frustration because of her mom was being very selfish and Satoko couldn't deal with it. She knew about the affair and wanted to keep the family together no matter what but it became futile in the end. I don't think she was sexually abused because her dad was a decent guy just a severe workaholic which caused him to neglect his duties as a parent and husband. The mom felt lonely and unloved so she started seeing someone which led her to neglect her duties as a parent too. Though I feel even before the affair she was a bit of a shitty parent. If Satoko was abused then that would mean she met the guy her mom was seeing or it was someone else but I just think she saw what was on her mom's phone and assumed or either confronted her mom about it. The wallpaper was put in the bathroom to hide the damage to the wall, to give the appearance that things were okay.
Thank you!! I appreciate the explanation ^w^
Since the guy her mom was seeing was being incredibly creepy towards Mayu, it gave me the impression that he would have no qualms targeting her or Satoko, and Satoko's behavior seemed to match with victims of abuse. I assumed when Mayu looked through the hole...she witnessed what happened.
I'm kind of confused on the background of Satoko here...was her mother cheating on the father? And what was the thin wallpaper thing about? Please don't tell me she was sexually abused...