[Undertaker]

Originally Posted by karice67

Actually, what Hoodspirit said (about Japan anyway) is accurate.

Spoiler for from the Japanese wiki:

現在日本には近親者同士の合意に基づく性的関係についての規定はない。強姦の場合は強姦罪、子供の場合は児童虐待防止法などの法律で対処する。日本の法律では直系の血族と、傍 系の血族で三親等以内の人との結婚が禁止されている（民法第734条および第740条）。たとえば、自分自 身の兄弟姉妹との関係は、直系ではなく傍系ということになる。法律的には、甥・姪の子どもやいとこは傍系4 親等で結婚可能。

In Japan today, there are no provisions against incest as long as the parties concerned consent to the relationship. If rape is involved, it's treated as a rape crime; if children are involved, it's dealt with using anti-child abuse laws. Under Japanese law, marriage is not allowed between blood relations that are related in a direct line or within three degrees for collateral relations (c.f. provisions no. 734 and 740 of the civil code). For example, you and your own siblings are related collaterally, not linearly. By law, you can marry the children of your nephews and nieces, or your cousins etc because they are fourth degree relations.

So if Kyousuke and Kirino want to have a relationship ONCE THEY ARE BOTH ADULTS, then the law wouldn't prevent them from doing so, provided they don't try to get married. (I'm not entirely sure which 'definition' of adult they use though. Japanese people come of age at 20, but the national age of consent is 13, and different jurisdictions can have their own ordinances that restrict it further, up to the age of 18 (source).)

@Sumeragi
Rather than 6th cousins, you and your fiance are 2nd cousins (related by 6 degrees collaterally), and Korean law prohibits marriage to the level of 3rd cousins (related by 8 degrees collaterally). I seem to remember you using the correct terms somewhere else in the forum though...

My bad, I was concentrated on the marriage side of thing. I stand corrected, thanks.

Other than that I am aware that the age of consent in Japan is 13 and in addition they can get legally married at age of 16.

As for the degree system. As karice67 said, yo are guaranteed 50% of your parents's DNA, but those 50% might not be the same 50% as your siblings. Granted, probability-wise you and yoursibling is more likely to have more matching DNA than your parents, but that's not guaranteed and that why the degree system is still works more vast majority of time.