Is that a smirk on his face? I can't really tell. If it isthen what a terrible brother...just like the rapists.
I want to believe that its just the perspective and he actually has a more embarrassed expression and not one of lust. Then I would think that he may genuinely want to help his sister. HOWEVER, given his dialouge in post #1044337 "I was thinking only about myself", its hard to tell so far what his intentions really are.
I personally think he's been struggling with himself. Part of him wants to help his sister, but the other part of him just wants to have sex with her. I'd assume that when he was forced to rape her, he found that he enjoyed it and (despite being ashamed of himself) he wanted more.
And I'm not the person to ask if there is a plot, but I doubt there is.