For a look at the genetic issues of inbreeding, see here
For the Westermarck Effect hypothesis, see here
Social and Ethical Reasons why Sibling Incest is Discouraged
There are social reasons for sibling incest to be stigmatized, even when considering a case of Not Blood Siblings. It is difficult to consider incest apart from issues of sexual abuse and whether consent is possible, since there may be a difference in age, maturity, or power between two siblings similar to the disparity inherent in Parental Incest. Even if two siblings mutually pursued a seemingly consensual relationship with each other, it would be dangerous if they were not of an age to understand the consequences of their actions. There may be a tendency not to permit any exception that might be taken advantage of by abusers, and not to limit the right of parents to make decisions about their children, so perhaps it's not surprising that a society wouldn't recognize such a thing as consensual sex between siblings.Even if we assume good intentions, romance creates different kinds of tension and conflict compared to the disagreements between family. When two lovers undergo a nasty breakup they can get out of each other's lives and fall back on friends and family, but if the same were to happen between siblings it could ruin the familial bond between them, perhaps irreparably, like a familial version of I Don't Want to Ruin Our Friendship. Any sibling couple who insisted on staying together would also face extreme social ostracism to the extent that one would be forced to wonder whether this relationship is worth the risks and future implications.
Another factor is that, although the extent of such may vary from culture to culture, family members are expected to support and show affection to one another on the principle of familial love, which does not encroach on romance. However, in families where incest is practiced (including parental child sexual abuse), spouses and significant others tend to see the perpetrator’s family members, including their own underage children, as romantic rivals. Thus, the incest taboo serves to preserve family bonds from being invaded by jealousy and abuse.
At the same time, different societies have varying ideas about legality, and the choice of which genetic or socialization factors to consider more harmful can be kind of arbitrary. Practically all jurisdictions punish rape or statutory rape of a family member. In cases of two consenting adults, however, it's more complicated. Some countries apply criminal penalties towards any sexual relations between siblings (Iran even has the death penalty as an option), while others such as Japan and Russia do not criminalize acts between consenting adults and merely prohibit them from marrying. Some only prevent blood siblings from marrying, while others also prohibit adoptive or step-siblings from marrying. It is critical to note consent is always dubious in the case of legalizing incest — "consenting adults" here can also refer to a parent and child over 18.
Certain thought experiments may turn out very differently depending on the legal jurisdiction and culture. For example, if two unrelated children were raised as siblings in an Oblivious Adoption, they could conceivably fall in love while believing that they were blood siblings. These feelings would carry all the social and psychological weight of real incest. But if the truth were revealed, they would be legally able to marry and have children. This doesn't necessarily solve any of the aforementioned social, familial, and psychological problems.
Conversely, if two long-lost siblings met and fell in love without realizing they were related, then Surprise Incest could derail their relationship or create pressure to split, despite them having never been siblings in any social or psychological sense. Even if they were to abstain from direct procreation, people who knew that they were siblings would find their relationship wrong on principle.
