So the European Court for Human Rights held in Case of Stübing v. Germany (Apr. 12, 2012).
The case involved a brother and sister who were reared apart for their whole lives; the sexual relationship began when the sister was sixteen and the brother seven years older, though sixteen is above the age of consent in Germany. The sister was described as having some mental difficulties, as well as “mild learning disabilities”: “The accused, K., has a very timid, withdrawn and dependant personality structure. This personality structure, taken together with [an] unsatisfying family situation, led to her being considerably dependant on the applicant. In particular, after the death of their mother, she experienced this dependency to an extent that she felt that she could not live without him.” I can’t speak to what extent this “dependency” is comparable to the dependency that many people who are in love (whether healthy or unhealthy love) feel, and to what extent it was dramatically more severe.
I tentatively think that adult sibling incest should indeed be forbidden (though perhaps with a higher age of consent), but I think the question is not easy, especially in constitutional regimes — like the European one — that generally protect sexual autonomy. Unfortunately, the European court’s explanation didn’t go into much detail; the core analysis seems to be in these paragraphs:
61. Applying the principles set out above to the instant case, the Court observes that there is no consensus between the member States as to whether the consensual commitment of sexual acts between adult siblings should be criminally sanctioned (see paragraphs 28-30, above). Still, a majority of altogether twenty-eight out of the forty-four States reviewed provide for criminal liability. The Court further notes that all the legal systems, including those which do not impose criminal liability, prohibit siblings from getting married. Thus, a broad consensus transpires that sexual relationships between siblings are neither accepted by the legal order nor by society as a whole. Conversely, there is no sufficient empirical support for the assumption of a general trend towards a decriminalisation of such acts. The Court further considers that the instant case concerns a question about the requirements of morals. It follows from the above principles that the domestic authorities enjoy a wide margin of appreciation in determining how to confront incestuous relationships between consenting adults, notwithstanding the fact that this decision concerns an intimate aspect of an individual’s private life....
63. The Court observes that the Federal Constitutional Court, having analysed the arguments put forward in favour of and against criminal liability and relying on an expert opinion, concluded that the imposition of criminal liability was justified by a combination of objectives, including the protection of the family, self-determination and public health, set against the background of a common conviction that incest should be subject to criminal liability. The Federal Constitutional Court considered that sexual relationships between siblings could seriously damage family structures and, as a consequence, society as a whole. According to the court, criminal liability was further justified by reference to the protection of sexual self-determination. By addressing specific situations arising from the interdependence and closeness of family relationships, section 173 of the Criminal Code could avoid difficulties in the classification of, and defence against, transgressions of sexual self-determination in that context....
65. The Court considers that the above-mentioned aims, which had been expressly endorsed by the democratic legislator when reviewing the relevant legislation in the 1970s (see paragraph 46 above), appear not to be unreasonable. Furthermore, they are relevant in the instant case. Under these circumstances, the Court accepts that the applicant’s criminal conviction corresponded to a pressing social need.
The German court’s decision, which was affirmed by the European court, is summarized here. The summary offers a slightly more detailed analysis; here’s an excerpt:
The essential ground considered by the legislature as the reason for punishment in § 173 StGB is the protection of marriage and the family. Empirical studies show that the legislature is not acting outside of its latitude for assessment when it assumes that incestuous relationships between siblings can lead to serious consequences damaging the family and society. Incestuous relationships result in overlapping familial relationships and social roles and, thus, can lead to interference in the system that provides structure in a family. This does not correspond with the image of family that is the basis of Article 6.1 GG. It seems conclusive and is not far-fetched that the children of an incestuous relationship have significant difficulties in finding their place in the family structure and in building a trusting relationship to their closest caregivers. The function of the family, which is of primary importance for the human community and which is at the basis of Article 6.1 GG, would be decisively damaged if the required structures were shaken by incestuous relationships.
To the extent the criminal provision is justified by reference to the protection of sexual self-determination, this objective is also relevant between siblings. The objection that the protection of sexual self-determination is comprehensively and sufficiently protected by §§ 174 et seq. StGB (crimes against sexual self-determination) and, therefore, does not justify § 173.2 sentence 2 StGB ignores the fact that § 173 StGB addresses specific dependencies arising from the closeness in the family or rooted in family relations as well as difficulties of classification of, and defence against, encroachments.
The legislature additionally based its decision on eugenic grounds and assumed that the risk of significant damage to children who are the product of an incestuous relationship cannot be excluded due to the increased possibility of an accumulation of recessive hereditary dispositions. In both medical and anthropological literature, which are supported by empirical studies, reference is made to the particular risk of the occurrence of genetic defects.
The challenged criminal provision is justified by the sum of the comprehensible penal objectives against the background of a societal conviction effective to date based upon cultural history regarding the fact that incest should carry criminal penalties, which is also evident in international comparison. As an instrument for protecting sexual self-determination, the public health, and especially the family, the criminal provision fulfils an appellative, law-stabilising function and, thus, a general preventive function, which illustrates the values set by the legislature and, therefore, contributes to their maintenance.