You don't seem to understand the question..
OK, you're related. What about being related is inherently bad with being intimate as long as it's between consenting adults? Remember, you can't use religious values, and you can't say because the genetics could possibly be damaged, because that's the example I already used, and that's what justifies the legal consequences. Saying "it's your sister!" is just like defining a word with the same word.
There is an actual name for that, in clinical psychology, when siblings are physically attracted to each other. Genetic sexual attraction.