
I mean the setting is fantasy in a medieval period and if you look into real life history it was normal for branch family members to get married in many places in our world, usually forced but rarely not, my guess is the author wanted to add traditions that were normal for that time, like underage marriage, slaves, etc.

personally, it's a no for me. It's not because they're like brothers. Honestly, some of them I have never met (they live abroad and our vacations never coincide) so they are as distant as strangers in that sense. For me it's 2 things:
- Don't want the probability of any birth defects even though studies show that the defects only show after multiple generations on marriages within the same family rather than a one off.
- Marrying a cousin feels like too much family involvement. My idea marriage is one where parents don't get too involved in the couples life, live far and meet occasionally. If my husband's parents try to get too involved I would tell them to back the fuck off (my parents wouldn't be a problem since they, themselves strongly believe in this). But it feels too complicated if it's my aunt and uncle (my husband's parents) and the shit storm involves the entire family...
It's too complicated for something not that worth it. It's not like there are no good men outside of the family.
But it's worth saying that the second point is what encourages some people to marry within the family (not necessarily cousin but maybe very distant cousin that shares the same background as them and their immediate common ancestor was 200 years ago or something like that). Some people like it when the big family gets closer. Not me though lol
its not that i hate the manga or anything its just that the fact were eris and rudy get married is disgusting i mean first of all they are cousin's and its just my opinion its not love at all