At their home in Leipzig, Patrick Stuebing and Susan Karolewski are in the kitchen, playing with a young toddler.
By Tristana Moore
BBC News, Berlin
They share a small flat in an east German tower block on the outskirts of the city. It looks like an ordinary family scene, but Patrick is Susan's brother and they are lovers.
"Many people see it as a crime, but we've done nothing wrong," said Patrick, an unemployed locksmith.
"We are like normal lovers. We want to have a family. Our whole family broke apart when we were younger, and after that happened, Susan and I
were brought closer together," he said.
Patrick, who is 30 years old, was adopted and, as a child, he lived in Potsdam.
He did not meet his mother and biological family until he was 23.
He travelled to Leipzig with a friend in 2000, determined to make contact with his other relatives.
He met his sister Susan for the first time, and according to the couple, after their mother died, they fell in love.
"When I was younger, I didn't know that I had a brother. I met Patrick and I was so surprised," said Susan, who is 22.
She says she does not feel guilty about their relationship.
"I hope this law will be overturned," Susan said.
"I just want to live with my family, and be left alone by the authorities and by the courts," she went on, in a hardly audible voice.
Patrick and Susan have been living together for the last six years, and they now have four children.
The authorities placed their first son, Eric, in the care of a foster family, and two other children were also placed in care.
"Our children are with foster parents. We talk to them as often as possible, but the authorities have taken away so much from us," said Susan.
"We only have our little daughter, Sofia, who is living with us," she said.
All but one of the couple's children have been taken into care
Incest is a criminal offence in Germany. Patrick Stuebing has already served a two-year sentence for committing incest and there is another jail term looming if paragraph 173 of the legal code is not overturned.
The couple's lawyer, Endrik Wilhelm, has lodged an appeal with Germany's highest judicial body, the federal Constitutional Court in Karlsruhe, in order to overturn the country's ban on incest.
"Under Germany's criminal code, which dates back to 1871, it is a crime for close relatives to have sex and it's punishable by up to three years in prison. This law is out of date and it breaches the couple's civil rights," Dr Wilhelm said.
"Why are disabled parents allowed to have children, or people with hereditary diseases or women over 40? No-one says that is a crime.
"This couple are not harming anyone. It is discrimination. And besides, we must not forget that every child is so valuable," said Dr Wilhelm.
The couple's case is controversial and it has prompted a heated debate in the media.
"We need this law against incest in Germany and in the whole of Europe," said Professor Juergen Kunze, a geneticist at Berlin's Charite Hospital.
"It is based on long traditions in Western societies, and the law is here for a good reason," said Prof Kunze.
"Medical research has shown that there is a higher risk of genetic abnormalities when close relatives have a child together. When siblings have children, there is a 50% chance that the child will be disabled," he said.
Patrick and Susan say they have no other choice but to fight the current law.
"I have read that some doctors claim that children born to siblings could be disabled, but what about disabled parents who have children, or older parents?" asked Patrick.
"People have said that our children are disabled, but that is wrong. They are not disabled," said Patrick.
"Eric, our eldest child, has epilepsy, but he was born two months premature, he also has learning difficulties. Our other daughter, Sarah, has special needs," Patrick said.
The couple claim they have received a lot of support from friends and neighbours.
"When we go out to the supermarket, people recognise us and many have told us that they support our legal challenge," said Patrick.
"We would like society to recognise us, as any other normal couple," he said.
In 2004, Patrick voluntarily underwent a vasectomy.
"It's legal for the couple to live together, and to share a bed. But they are breaking the law once they have sex. If there are no more children, then who will be able to prove that they are a couple?" asked their lawyer.
Dr Wilhelm said a ruling was expected in the next few months.
"We've already heard that the vice-president of the Constitutional Court said that there will be a 'fundamental discussion' about this issue in Germany," said Dr Wilhelm.
"Many criminal law experts say that we are right and I'm confident that my clients will win their case. The law against incest is based on very old moral principles. The law was abolished
in France, it's about time it should be scrapped here in Germany as well."