Dunno how he's gonna reenact this, but I was reminded to the scene in the car when Sei was talking to his Aunt and she explained how his mother would do everything for him. I wonder if she knew Sei had intentions or feelings of pushing Shigeru and decided to do it. In addition to that, maybe she helped him in killing the cat or picked up on his "tendencies" way back at that age - 'tendencies' referring to either Sei's possible violent tendencies or to Sei's mother doing things for him.
Really got me to think more about the mother's character and about life as Sei was growing up as well as how Sei is going to act this out, if he's going to push Shigeru off in this reenactment. Heck, I could also see him pushin his mother off! Either way, we're gonna be seeing what sort of person Sei is in this next chapter.