>>720648963
Whatever isn't beneficial to the long term prosperity of a society is deemed degenerate, doing that action would lead to the downfall of that civilization. If sister-fucking became the norm, it would lead to a decrease in genetic variability and inbreeding, with people being less likely to seek partners that aren't related to each other. This is, most likely, why we've developed this ick towards doing our close relatives, be it due to societal disapproval or our biology.
Ultimately, besides that, I guess fucking your sister in an individual level doesn't warrant any consequences unless you get them pregnant, which can still happen even if you use protection. Incest with relatives that aren't as close is fine, though:
cousins can have children as long as their children doesn't inbreed, second cousins can do whatever, they're closer to strangers than relatives.
