I know for a fact that Josh is a half (I can't find my copy at the moment, but I believe that's mentioned in...The Lunatic Cafe? Whenever Anita and Ronnie were x-mas shopping). I'd managed to forget about the sister entirely, but isn't she a step sibling? From Judith's first marriage?

Obviously Anita only knows about relationships as far as Laurell knows about them, and since Laurell doesn't see the difference between half-siblings and step-siblings, why should she bother finding out?