When somebody has sex with a member of their own family. When in doujinshi or fanfics, when two characters who belong to the same family, do such a forbidden action seen as wrong by many, but consensually because they are in love with each other, it automatically hyperboosts the quality of the doujinshi/fic. In real life, there is a level of controversy regarding incest. People in America can't do it, even in a consensual manner, by law, the British Royal Family has to do it by law, and some places probably don't even give a fuck whether you do it or not, as long as both partners wanted to do it, therefore no one being raped. When a lot of people think incest, they think deformed children and rape. But, birth control + consent of both partners = the right to declare a stereotype that anti-incest laws revolve around, false. Incest = redneck? Not necessarily. Anyone from any walk of life could be practicing incest. If somebody falls in love with a family member, you know sex will most likely be involved, therefore stereotype #2 has been declared false.
If they both are in love with each other, ain't a damn thing wrong with it, is there?