Germany's highest court has upheld the law making incest a criminal offence, rejecting an appeal from a man who has had four children with his sister.
Patrick and Susan did not know each other as children
The court ruled that the state was within its rights to protect "family order" and prevent the serious genetic illnesses that could arise from incest.
The man, identified as Patrick S, has already spent time in prison for incest but has been free pending his appeal.
He was adopted and only met his sister in 2000 when he met his birth mother.
The Federal Constitutional Court said the law against incest was in line with Germany's constitution.
One judge dissented, however, saying the punishment must match the offence and that the law "is based exclusively on moral beliefs rather than with the objective of legal protection".
Patrick S has served more than two years of the two-and-a-half year sentence he was given in 2005. He now faces the possibility of returning to prison to serve out the remainder.
All but one of the couple's children have been taken into care
His sister, Susan, was also convicted of incest and placed on probation for one year.
Patrick's lawyer, Endrik Wilhelm, argued that the law against incest was a "historical relic" and the couple were doing no harm to others.
But the court said that sexual relations between siblings "do not affect them exclusively, but also can have an effect on family and society and have consequences for children who arise from the relationship".
Patrick S, from Leipzig, was adopted when he was four years old and met his sister in 2000 when he tracked down his birth mother.
He was 24 at the time and she 16.
Their four children are aged from six years to 34 months. Three of them are in foster care.