The European Court of Human Rights affirmed a three year incarceration sentence of a German court in a "consensual" brother-sister incest case.
Notably the case wasn't an entirely clean case of consent, as the sexual relationship, commenced when the sister was sixteen and the brother was twenty-three, showed indications of undue influence, made possibly precisely because of the trust they placed in each other as a result of the brother-sister relationship, exerted by the brother over a young woman with diminished capacity who was grieving the death of her mother. These facts help explain why it might be fair to punish one but not the other, although both objected to the prosecution. The case addressed has some similarity to a prosecution under Colorado's sexual assault on a child by one in a position of trust (Colorado Revised Statutes 18-3-405.3, which would be a class 3 felony on these facts if the statute of limitations had not run, which it probably would have in this case after four children were born) which carries a much longer minimum and maximum sentence of incarceration (or under the incest statute, which would be a class 4 felony on these facts). According to the European Court, using the word "States" in the sense of national governments, "twenty-eight out of the forty-four States reviewed provide for criminal liability" for brother-sister incest between adults.
The siblings were raised apart during their childhoods. The relationship would not have been statutory rape in Germany if they were not related and it does not appear that there was a finding that the sister was so mentally impaired that she would generally be incapable of consent. They had four children, three of whom have been placed in the German equivalent of foster care. Two of four children had disabilities, although given the sister's mental impairment, this could have been simply a matter of inheriting the issues that she already displayed that might not have been mitigated in another relationship.