Conclusion
The fact that Rita was turned on to the basement orgies and enjoyed watching the sex from a secret hiding place cannot be blamed for what happened with her brother. That merely gave Steve an opportunity to gain domination over his sister.
An incestuous relationship that goes on over a period of time can be a very tenuous and sometimes dangerous thing. At this point it is important that Rita and Steve go for therapy together. If, as the girl says, she enjoys the fucking with her brother, she should continue it. But it is obvious that she has ambivalent feelings about her brother that stem from their intitial childhood rivalries.
It is important that this girl accept the fact that women and girls do enjoy watching sex and that there is nothing for her to feel guilty about!