Oh shit, is the mom still alive? Was this all to break the son?????
I might be mixing up fan theories with canon, but wasn't it clearly stated that the brother killed their mother to put an end to her madness?I'm thinking that either the son himself or someone else who held a grudge arranged for her car crash, and murdered her and hid the body, or somehow, she is such a fucking lunatic, she faked her own death in order to break her kid as hard as possible after she realized he wasn't completely dead inside due to his sister.
We do now know he was the one who left Yotsuha at the orphanage, so killing the mother was his last resort to save his sister.
Feel free to correct me if I'm wrong. It's been a while, so I'd need to reread the manga to be sure.