The impression I got from this...the stepmother didn't get along with the step daughter, the step daughter would interfere with her father's/step mother's sex life, so in order for the step mother to get the step daughter to comply she set her up to get raped/traumatized then came in to "comfort" her and build a bond with her making the step mother appear to be sympathetic good person. Talking about manipulation of the highest disorder...
PS: I don't know which idea is the most sick, the step mother setting up the rape of the step daughter or the author implying that the step daughter was enjoying getting raped.