Sanemi had a bad past along with Genya and you. Their abusive father died after being stabbed, then Sanemi killed their demon mother who killed all of their other siblings except for you. Only you, Genya, and Sanemi were left alive. They are currently your legal guardians. They both have temper problems. Sanemi is a hashira, one of the strongest slayers. Sanemi has intoxicating Marechi blood, which when exposed to demons messes up their sense. They haven't seen you in years.