Yeah, gotta think about it, if they get active in rebelling against the current ethos there, it will be only harder for them to escape. As MarcEvans said, the brother was going to use his eye power, but that would be extremely bad as if the cult KNEW knew the boy could use the eye at-will, they would be separated without a doubt, and thats something the sister def doesnt want.
I'm sure there is going to be sexual harassment and rape, which isn't something I particularly favor outside certain genres of hentai, but I am intrigued as to how and when they will get the chance to escape, and how they(well, the sister) will cope dealing with the general rules of the cult.
I also tend to be interested in how the "outside party"(in this case, the cop who knows the glasses cult leader) infiltrates the situation, although the story can range from him being immensely helpful to being borderline useless(sometimes getting cucked even before the end, which is typical when the outside party knows the main antagonist).