OK, just adding my two cents here... Even disregarding the problem of morality and genetics (which he doesn't really know about, having lived a life of social isolation from young age (blaming the parents on that...)), there is the matter that:
1) he doesn't see his sis as a sexual or even a romantic partner
2) he felt trapped by what his father had done to make him marry his sister, right after him realizing he's got NO. FUCKING. HELP. In his predicament...
So... Please get over yourselves and accept that he doesn't want to, and that he couldn't talk his way out of an arranged marriage. As someone said before, if the genders were reversed, y'all would be cheering for the MC for running away from a fucked up situation.