I understand that the goal is to get laid.. this is a Hentai fantasy after all. So it is a matter of the main character putting up with whatever they must to accomplish this. I also understand that you must try to write the characters as close as possible to how they are presented in their source materials as possible.
It would make more sense if the main character could "break" Asuka, possibly turning her into a dependant sub. He has only known his stepsister for the duration of the story, thus he has only known her to be abusive and manipulative. Nothing (even the tub scene) shows her to be redeemable. Nothing has occurred to show he would ever have reason to care for her. She only uses sex as a method of control as opposed to actual intimacy that might suggest the possibility of redemption. This would be magnified by the fact that he knows she is a fictional character and only code that could be re-written to become a better "person".
There might also be a potential path available for change, if one looks to Jessica Rabbit.. to paraphrase, she isn't bad, she's just written that way. But she would have to not only accept that the simulation is a lie, but that Everything she knows about herself is only fiction.. there was no father to leave her, no giant mechs to pilot, none of her previous experiences ever having happened. This would essentially kill the personality known as Asuka Langley leaving her a blank slate with red hair and language skills.
Just my opinions.