@gamria makes a good point. There has to be something that happened before Genya finds his mom dead and Sanemi just standing there looking shocked.
Since Sanemi's blade isn't bloody and not a nichirin blade, he can't be the one to kill her. Even if the sun is the cause of her death, shouldn't she be in ashes instead? Muichirou's past mentions his demon turning into ashes right after Genya's past so it's probably intentional for Gotouge to leave this an open case still
lol like am i the only one who LIKED sanemi and genya from the beginning?! Also i have this feeling that the reason why sanemi told genya to leave the demon hunters is because he doesn’t want him to get hurt ¿