Zac needs a step sibling, so if you were up for it, it's on the table- details of his step-father or his step siblings haven't been mentioned whatsorever, so that would all be up to you- the only thing set in stone is that zac's parents divorced when he was 11 and his mother remarried when he was 13 (4 years ago).
Even if you choose not to fill the line, I'll offer him up for something.