Abusers don't start out that way. SM appears to be pretty narcissistic. She would have her 'good' side, and could probably be very charming, kind, wonderful, loving, etc. I'm sure she had some qualities that charmed both he and the child.
Power and control are what fuel abusers, and if she was truly narcissistic, she probably put people up on pedestals, and when they fell, they fell hard. She might have liked Zahra, so long as Zahra 'played by her rules'. As the child showed she was more of an individual, she might have begun to sour on her, til she was taking everything out on the child.
Dad might have been charmed too, and I believe he might have endured abuse, either physical, mental, emotional, etc at hands of SM. Abuse has cycles, and he wouldn't always be abused, either. Men can and are abused...I just had a case with one recently. The thing is, sometimes, after a person has been abused so they begin to 'idenitfy' with the abuser. Its sometihng they do subconciously in order to survive, (think how hostages sometimes have this happen).
This could be combined with dads work. He stays away as much as he can from the house, to make more money, but really because it's his only escape. He may not want to know what's happening, and may deny it. I think, if it were too late, (child was murdered by SM), SM would tell dad it was his fault, because of something he'd done, and if he didn't help her, she would make sure the police knew it was him....something along those lines.
This is just my opinion, and just a thought.
That may just make him irresponsible not brainwashed. I don't think he could be brainwashed before he came to the states maybe he was infatuated with this woman and came here for himself not caring about poor Zahra.