
The biggest point is: How smart is she? Is she a dumb victim that is going to be humiliated and eventually raped or is she a cunning person that is going to use any means, including sex, to survive and get revenge? The only way for her to escape both is to be saved by an external influence, and that doesn't seems to be the story that the author wants to tell. So this manga is either about her silent suffering or about how she will fight back. Since she is utterly powerless the only tactic she can use is "divide and conquer" by making the members of the family fight among themselves and for that she will have to manipulate them using the only weapon she has: her beauty. So maybe even if she does end being raped it could be a part of her game. Either way she is a pitiful girl that could have been bright and happy.
Thanks for the link to the missing pages.
If nothing quickly happens to change her situation she will be soon raped by one or more of the step brothers. Quite dark for a cinderella story.