I have several point to say:
1. Anyone who said that romantic siblings relationship is wrong, are right BUT this is a fantasy world where that normally accepted and become custom. Just because this is isekai you can't force our world way to the story. If the story is pure fantasy(without isekai thingy) no one would complaint, I guarantee.
2. Mc run away because he doesn't want marriage his sister because its fells so wrong for him. But in other hand he want his sister become more socially friendly when the only one who have interacted with her is him, and want see her family in the future without even talking about marriage, his thought and her feelings for him if he want marrage cancelled, he can make a proposal to postponed the marriage with the excuse he feels not ready, still confused or whatever. And in this time he can make her to more open to others especially boys if he want her marriage other person. I don't know who will be her partner, fatty/ his close friend or other boys because there seem no many kid in the clan.
3. He worried if he will accepted again by the clan in the future because, but doesn't care his parents, sister and the other clansman will thinks about his family, does he become disgrace to family or even the clan? Does that make his family happier without him? certainly NOT.
4. Last, if he leave the clan that means the asshole kid will monopolies weed again right ?
Conclusion :
1. MC is fkcing egoistic asshole who only thing ME ME ME and ME
2. Mc is so fucking loner can't even talk his life problem to his parent, spool self insert to loner/ hikkomori problems in Japan.
3. author rushing the plot for his novel description with time skip( we can see this in the manga even ln), to complete the prologue and start generic slow life like the title says