The 29-year-old brother has already spent more than two years in prison for sleeping with his sister, and could be incarcerated again, his lawyer said.
The pair are currently drawing up an appeal to take before Germany's constitutional court.
They argue they are being denied the right to sexual freedom.
The pair did not grow up together. Patrick, the brother, had been adopted by a couple from Potsdam while his sister, Susan, grew up with their mother.
But when he reached adulthood he wanted to track down his biological parents. He found his mother, and he met Susan.
About six months later, the mother died, and Patrick moved in with Susan. In 2001, their son, Erik, was born. He was subsequently taken into foster care.
Three more children were born between 2003 and 2005. In November of that year, the pair were tried for incest.
Patrick was sentenced to two-and-a-half years in prison. His sister, then 21, was put into the care of youth services.
According to Spiegel online, while Patrick was in prison Susan had a fifth child with another man.
Their lawyer, Endrik Wilhelm, told the BBC News website that the pair are now living together again and plan to lodge their case within six weeks.
Story from BBC NEWS