In Japan, it's legal to marry your first cousin and is not frowned upon in the slightest (Shelbyville Manhattan would be proud). This contrasts how it is seen as incest in the west given the relative closeness in genetics. That's why such an emphasis is placed on finding someone outside your immediate family, to avoid inbreeding.What's with Brother's unhealthy obsession with Yuna? He does realize that she's his cousin right? And that's not even mentioning how inappropriate he can be towards his own sister.
Here's an educational video related to the topic.