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BERLIN - Patrick Stuebing was adopted and had to wait until he was eighteen before he could meet his real family for the first time. It was a fateful encounter that led to his life being torn apart by sexual love for his sister and more than two years in jail for committing the crime of incest.

Yesterday Patrick, 29, and his sister Susan, 24, who have four children from their seven-year relationship, announced plans to take their case to Germany's constitutional court in a bid to overturn a century-old law which rules that incest is a criminal offence punishable by prison.

"We want the law which makes incest a crime, to be abolished," Patrick Stuebing, who faces the prospect of another jail term for continuing his relationship with his sister, told Germany's Welt am Sonntag newspaper yesterday.

"We do not feel guilty about what has happened between us," both added in a joint statement.

The couple's case has sparked wide controversy, not least because Germany, unlike many of its European neighbours, such as Belgium, Holland and France, continues to treat incest as a criminal offence.

Several German doctors have implied that the ruling is necessary to prevent the onset of illness caused by in-breeding. However, a growing number of politicians and legal experts have called for the law - which formed part of the "racial hygiene" policies of the Nazi era - to be scrapped.

"We are dealing with a piece of legislation which dates back to the last century and which no longer makes any sense," said Jerzy Montag, a spokesman for Germany's Greens party.

The case of Patrick Stuebing and his sister defies popular notions about incest because the couple did not know each other during childhood. Adopted at the age of four in Communist East Germany, Patrick was only allowed to go in search of his real family when he reached the age of eighteen.

His father had died in the interim, but he found his mother, Annemarie, and Susan in the east German city of Leipzig in 2000. Six months after the reunion Annemarie died of a heart attack. Patrick and Susan were the sole remaining members of a once divided family.

Drawn together in grief, the couple fell in love. "There was nothing we could do about it, " Susan said. They went on to have four children. All but one of them have since been placed in care and two are mentally damaged as a result of in-breeding.

In 2002 Patrick was sentenced by a German court to a year's suspended jail term after being found guilty on sixteen counts of "illegal coitus" with his sister.

The authorities placed their first son, Eric, in the care of a foster family. Two other children followed in 2003 and 2004. Patrick was subsequently jailed for ten months for failing to respect the terms of his suspended sentence.

Before starting his jail term, the couple conceived their fourth child, Sofia, who was born in 2005.

Patrick and his sister found themselves before the courts again in November 2005 on account of their subsequently born daughters and Patrick was sentenced to two and half years in jail for re-committing incest. His sister was placed under the care of a social services department.

Patrick, an unemployed mechanic, was released from jail last year and is still living with his sister. Although he has had himself sterilised to avoid fathering more children, he faces the imminent prospect of being sent back to jail for persistently re-offending.

In an interview published yesterday the couple insisted that they would not have had additional children if their son had not been taken away from them by the authorities. They said that all they wanted to do was to "live together as a family".

Endrik Wilhelm, the couple's lawyer who will take their case to the constitutional court, said yesterday that he believed they had a good chance of winning.

He pointed out that there was no law which forbids older people or those with hereditary diseases from having children although, as in the case of incest, both also carried the risk of subsequent mental illness.

"This is a case about family rights and the right to sexual self-determination. Patrick and Susan are not doing anyone any harm," he insisted.