Please note Seiichi did not open his mouth the entire sequence, and that his mother is also emotionally unstable, thus her narrative should be considered carefully.
Seiichi most likely did not DIRECTLY have any influence on the event itself, as it was his mother's delusions that made her convinced that pushing Shigeru was what Seiichi wanted.
Looking up butterflies in Japanese symbolism is particularly interesting:
[ul]"Symbol of womanhood and successful marriage" (rather ironic)
Represents "Cycle of life and death"
"Associated with challenging mystery or with problems leading to madness, so that following a butterfly may result in dispelling overwhelming doubts"[/ul]
The last one is most applicable to this scene, fluttering unsettling madness that covers ma's vision, constantly hanging around (really beautifully illustrated in this chapter too). Ma's insecurities of being a failure as a mother, a wife, as a woman has mentally deteriorated her mind and warped everything she sees. The way she sees the two children is not flattering. The way she sees herself is also old and tired and beyond the age she should be.
There's a lot of discussion on who the blame should be pushed on, and it's probably a difficult one to answer (as long as we are talking about this morally rather than legally). Please get therapy, both of you.
I'm not too sure how I feel about it in relation to this series as a whole (especially how it connects to the emotional abuse of Seiichi), but as a stand-alone chapter, I loved it. The perspective it brings onto the table is an absolute feast to think about, the way anxieties and paranoia is portrayed is sexy as hell, and gives this series so much re-read potential. Author, I am entrusting you with my life when I say I don't think he's just making twists for no reason.
edit: i fucked up, seiichi does "speak" but there's too many things pointing to that not being literal