Yeah, evil enough to seduce her brother into impregnating her and then spend the next twelve years catatonic in an asylum before killing herself just to traumatize the kid.
Oh, drool… what a hero, this actually impresses her.
For example in one episode Mitsuya sees Hikaru helping his sister then Mitsuya tells his sister to scream in order for the police to catch him as well as branding Hikaru as a lolicon for days.
Episode 19 explains that Himari was adopted into the Takakura family after she was rescued from a plance named the Child Broiler.