The parents mention how the MC is such a good child for actively following their clan traditions like learning magic and creation of totems which they clearly don't need to keep up anymore due to the war being over for hundred(s)? of years... Yet, he refuses to keep their tradition when it comes to marrying your own sister? Obviously we don't know the reasons why, which is why I am going with the beta self cuck male MC route at the moment...
But maybe, he just has romantic interests for another girl? maybe he already proposed to another girl? From the manga description and the first chapter we have no idea why he runs away from marriage with his own sister (as by culture demands), so I can only assume that he is taking his old morals from his previous world and deciding to get the fuck out, hey bro whatever floats your boat if the kid is allergic to pussy so be it
