we should fund an international "incest is wincest" movement, toward global sexual happiness.
Wars would end instantly, and birth rate would plunge, since a lot of males would be... redundant. But we're 6 billions so... Hey, maybe incest taboo is explaining this large number in the first place !
The saddest thing about this manga is that this incestuous relationship started off innocently; just a regular older sister trying to comfort her sick little sister. It turned sexual because of sexual awakening and teenage curiosity and nothing else, as far as the manga was portraying. It's sad because the mangaka is sexualizing a female sibling's relationship.
Also to note, incest is still taboo because it doesn't stop at same-sex parties. In most "reported" cases, it's largely of the opposite sex. So there goes your theory that "birth rate would plunge" and "incest taboo explains large number in the first place".
Let's also not forget your quote, "alot of males would be redundant". I'm not sure what you mean by this, but I'm guessing they won't have a female mate (because of lesbian incest)? Well, if they can't get them voluntarily then they can take them by force. Rape still exists, and unfortunately for many developing countries (where birthrate is highest), abortion is still illegal. Even if conception was the result of rape and incest. It's progressive to think liberally, but there must also be a boundary and a line drawn with issues which are considered "taboo" in the first place.