I understand that some people revolt at the idea of being romantically involved with your sibling and I don't deny that it's gross but is it really so bad to tolerate it? At first I wanted to hurl when I first watched the OVA but I slowly grew to accept it to an extent. But what I realized is that people don't seem to know how common incest was (or is). So I thought I might point this out:
I did research and in the early 21st century, two German siblings are married and had a child even though they had to serve jail time as a result of incest even the rest of Germany questioned this involvement because the couple did nothing wrong. And just recently, I read in a National Geographic article that it was common for Egyptian and Hawaiian royalty to have incestuous relationships. The father and mother of King Tutankhamun were siblings and consecutively King Tut married his half-sister Ankhesenamun. In Thailand, way back when (they didn't mention the exact date) King Rama V was husband to his half-sister as well. King Kamehameha III of Hawaii banned royal incest but that didn't stop him from continuously sleeping with his sister Princess Nahi'ena'ena. (This one paragraph is courtesy of National Geographic who helped save me from days of research.)
In fact, in Greek mythology (in some versions) the God of the Heavens, Uranus and his sister Gaia, the Goddess of the Earth were married and had children, two of them called Chronos and Rhea who in turn had Hestia, Poseidon, Hades, Demeter, Hera and Zeus. In Greek and Roman mythology Zeus/Jupiter, the King of the Gods and Hera/Juno, the Goddess of Marriage who are full-blooded siblings are also married and had many children even if Zeus/Jupiter was highly unfaithful. Even Poseidon, the Sea God pursued his sister Demeter, the Goddess of Agriculture at one point. The stories about the Legend of Camelot say that King Arthur was unknowingly seduced by his own half-sister Morgause and together they produced a son, Mordred who would be responsible for Arthur's doom.
In fiction, you have the Mortal Instruments series where the relationship between the central character Clary and Jace are incestuous for a while
In Shakugan no Shana, Tiriel and Sorath display public incest by kissing each other. In Vampire Knight, Yuki and Kaname Kuran are romantically involved. In Shugo Chara!, Utau Hoshina is in love with her brother Ikuto Tsukiyomi and forcefully kisses him even though he does not reciprocate her feelings.
until it is revealed Jace is not her brother.
I thought it would helpful because this manga is not the only thing breaking social boundaries and taboos.
"Imagination is more important than knowledge." - Albert Einstein