Best advice is to think like an egyptian.
the worst examples can plot and scheme for YEARS to get what they want.
PLEASE NOTE I SAY THE WORST EXAMPLES... Don't want to offend anyone, nor be called a racist. I lived in that country, I lived at the hands of someone cruel, manipulative and abusive.
Your sister is married to one of the worst examples. I was involved with a pretty poor specimen it has to be said, but more mental violence than physical, mostly. Though he did feel entitled to lash out and hit me on more than a few occasions.
Firstly, I assume all the others are out already? so the focus is to get the 8yo back, to retrieve the passports ideally, and to get the hell out. This can not now be done on a whim, it has to be planned, schemed and cooly executed. OP's sister needs to get her head into the coolest of cool places and be ruthlessly focussed on her objective.
With any luck she has an emergency place to go to if things get dangerous. If she can't get the DD to come to her via a taxi and then to the nearest consulate, then she will have to go back in, and resume what appears to be normal life. Could the sister go and get DD from school?
Ideally she needs to get the passports, but they can be re-issued by the embassy. I'm hoping they don't have egyptian passports too? Nationality?
get a flight bag together, and stored with someone safe for her and her DC, some cash if poss, otherwise tell her to send you details of a bank and get some money transferred over to her. If you have her details and her location, there could be a possibility that you could contact the consul and ask them for their help to go and get her to somewhere safe. I know the Consul in Pakistan goes and gets women that have been forced into marriage, why not to help women that are being violently assaulted? Every avenue is worth trying.
Backhanders might be the best way to get her out, sure it's illegal, but if she is being battered, she needs to get out for her own safety, and her DC too.
The consul will be the best first call, they will I hope be able to refer her to someone to help her legally and illegally with any luck.
Again, all the very best of luck to her and to you.