@jazz9 If you are in a group where everyone else is hunting animals for sport, it would be rude of
them to force you to kill, after all it's your choice.
But the MC's family never really asked anything else of him until the arranged marriage. He is indebted to them for living up to that point. And as everyone here knows, it's not the "marrying your sister" that is the problem, but the genetic defects that may occur in the offspring. Thus, he should have simply made the condition to his sister to never have children together.
Wikipedia says:
"Children of parent-child or sibling-sibling unions are at increased risk compared to cousin-cousin unions. Studies suggest that 20-36% of these children will die or have major disability due to the inbreeding. A study of 29 offspring resulting from brother-sister or father-daughter incest found that 20 had congenital abnormalities, including four directly attributable to autosomal recessive alleles."
But as other people have suggested, disability due to inbreeding might not even be a thing in their world. If he had made a study to estimate the risk of congenital abnormalities in sibling-sibling unions in his village's population he may have found that it's much lower. But no, the only thing he is able to care about researching is magic because... plot reasons.
Anyway, this is getting far too big brain to ask for from a manga such as this, so it's best to just drop the subject.
