Patrick Stuebing, who was adopted as an infant and met his sister in his 20s, has launched several appeals since being imprisoned for incest in 2008 and his lengthy legal battle has prompted widespread public debate.
Sexual relations between siblings or between parents and their children are forbidden under section 173 of the German criminal code and offenders can face years in prison.
But on Wednesday, the German Ethics Council recommended the section be repealed, arguing that the risk of disability in children is not enough to warrant the law and de-criminalising incest would not remove the huge social taboo around it.
The chairman of the council, Christiane Woopen, was among the 14 members voting in favour of repealing section 173, while nine people voted for the ban to continue and two abstained.
A statement released on Wednesday said: “Incest between siblings appears to be very rare in Western societies according to the available data but those affected describe how difficult their situation is in light of the threat of punishment.