Germany's highest court confirmed that the law that makes incest a punishable offence is in line with the country's constitution, saying that the law was designed to guard families against the damaging effects of incest and protect the "weaker" partner in incestuous relationships.

The court also took into consideration that children born from relationships between brothers and sisters had a greater risk of suffering genetic effects, the judges said.

The case was brought by 31-year-old Patrick S. from Leipzig, who has four children aged between six years and 34 months with his sister, Susan, who is eight years younger.

All the children but one are now in foster care. Two were born handicapped, although it is unclear whether this is a result of having been born prematurely or inbreeding. In 2004, Patrick S. voluntarily underwent a vasectomy.

A right to sexual relations among siblings

The couple were calling for a declaration that brothers and sisters have a right to sexual relations with one another.

Patrick S. has already spent over two years behind bars and now faces a 30-month sentence.

Susan has also received a conviction of incest and was placed on probation for one year. Under German law, incest is only punishable after the age of 18.

"Historical relic"

The couple have been backed by a number of politicians on the grounds that incest might be immoral but should not be punished by law because a ban contravenes an adult's right to sexual freedom.

Their supporters argue that the law is out of date and say it harks back to the racial hygiene laws of Nazi Germany. Their critics, meanwhile, insist that incest should remain a social taboo because of the risks linked to inbreeding and the resulting imbalance in social and family relations.

Endrik Wilhelm, the couple's lawyer, argued that the couple have caused no harm to others. The law was a "historical relic," he said.

"Everyone should be able to do what he wants as long as it doesn't harm others," he has said, stressing that incest is not illegal in many of Germany's neighboring countries, he said.

Growing up apart

Although they shared the same parents, the couple did not grow up together. Patrock S. was adopted as a baby and only met his sister in 2000 when he tracked down his birth mother.

According to German media reports, the couple became close after their widowed mother died at the age of 50. The relationship began when Susan was 16.

According to experts, incest is rare among siblings who have lived together, and usually occurs between brothers and sisters raised apart.

The phenomenon is known as genetic sexual attraction, and affects as many as 50 percent of siblings or parents and offspring who were separated at birth and reunited as adults.

The case has attracted vast media coverage in Germany, with the couple fielding ongoing requests for interviews, book deals and film rights.