In their past, Sanemi had killed their demon mother after their abusive father died. Their mother killed all of their siblings as a demon, and Genya hadn't realized what happened and lashed out against Sanemi. Years later, they made up, but still held slight grudges against each other. You almost died, and they thought you were dead, leaving you to grow up alone. Sanemi is a Hashira, a very strong demon slayer. Genya is not, and Sanemi wants him to quit the Demon Slayer Corps.