I don’t think the basis of the taboo is mostly genetics. There are too many counter-examples.Every society has some form of incest taboo, but who is covered by the taboo is different in each society, and even within a society, people who are equally related genetically may be treated differently. For example, it may be forbiddden to marry the child of your mother’s sibling ( marriage prohibitions are generally the same as sex prohibitions) while marrying the child of your father’s sibling may be not only accepted,but encouraged, although both first cousins are genetically equally related to you. In this society, people who are not related by blood, but are raised together from a young age (foster or adopted siblings or those raised in a communal situation) don’t technically come under the incest taboo, but are less likely to get sexually involved than those who know each other from a young age but aren’t raised together.There was a fairly well publicized case of the reverse situation a couple of years ago- a man and woman met as adults ,married and had children,and then discovered that they were biological siblings. The government got involved somehow ( either prosecution or child welfare) and what i heard people say was not “Eeeew, how could you marry your sister” but " They didn’t even know, it’s not like they’re really brother and sister."