@GalladeGuy understandable, which is why we have the option to discuss our differences in opinions.
For me personally this is how I view the situation.
Let's forget that he is a reincarnated individual from another world for a bit. I see this as a difference in cultural society as well as morals. For example, imagine you go to the middle of a jungle and find a tribe that has been living off the jungle for thousands of years with absolutely zero communication with the modern world.
Who exactly am I, to come to their society their culture and tell them how to live their life? Maybe they do have incest in the tribe like the clan in this story, I don't know... I am a guest in their world so I will obverse and do as told.
I don't believe I have the right to bring my own beliefs,morals, and "laws" to this tribe in the middle of the jungle which have clearly lasted way more than what we have.
Now, back to the story and some spoilers for chapter one.
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 😎
The character design is really good, but after i read the synopsis it kinda dissapointing.
I mean sure in a sense if he ran because of his old moral sense it's kinda understandable. But still, if it's a tradition that ran for hundred of years to be married into your own sister (which is nice, and his sister is a cutie too) and he decided to ran away without saying anything or he is an actual beta male MC, i don't think i can keep up with the story.