For all those sadly affected

Sadly, criminal law in Japan does not recognize self-defense in the Western style. Basically it amounts to the injuries sustained on either side, rather than the reasons for the fight or assault. In this case, the mother suffered the worst injuries, so the younger brother would likely be charged as a full-blown criminal. Japanese self-defense mostly relies on the victim either running away, or calling the police before fighting back, even then it is ambiguous. For most Westerners, this is tantamount to letting any aggressor win the fight or fire the knockout blow before the victim can react. The victim cannot prove that any physical retaliation was in genuine self-defense unless there was a witness (unlikely).

It is humiliating and short-sighted, narrow justice, but this is how it works in Japan.