If crazy mom ends up in jail it can only be a good thing for Sei. Even with only a very disinterested father like that one, even in an institution for kids without parents, it would still be better than having that psycho as a parent in the same house. The only problem now is whatever craziness Sei is gonna do.
edit: oh, maybe he's gonna say he pushed him off to cover for crazy mom. I don't know just how much the testimony of a minor is evaluated in Japan's legislation though.