Seeking legal help
French national J-L's brother has separated from his Japanese wife.
"According to Japanese law, my brother's visa ended a week ago. However, they have two young kids who would like to live with their father. According to Japanese policy, if my brother returns to France, he won't see his children again. What shall we do?"
First, head to your embassy and seek assistance. Hopefully staff can point you toward a French lawyer with knowledge of Japanese law.