Fatal attraction of siblings
Imagine what they thought, imagine the turmoil they endured.
They are the twins who, it emerged on Friday, were separated at birth and given up for adoption only to meet by chance years later - and marry.
The man and woman, unaware they were brother and sister, had grown up separately, perhaps far apart, in different families.
Yet when fate brought them together again, they experienced an uncanny bond and a sexual attraction.
As Lord Alton, who revealed the case, said: "They were never told they were twins. They met later in life and felt an inevitable attraction."
Did they sense some blood relationship? It certainly must have seemed odd: both had been born on the same day in the same year. Did they just take that as an incredible, happy coincidence? Did they not know they were adopted? Or did they suspect they were related, only for the power of their attraction to prove overwhelming?
According to Alton, who was told about the case by a judge, the couple married and only later discovered that they were twins.
"The judge had to deal with the consequences of their marriage, and all the issues of their separation," he said.
"For them it was a terrible tragedy. It was an incredibly heart-rending experience".
The marriage was annulled at a special hearing in the London high court last year, with the judge ruling it had never been valid. Under the 1986 Marriage Act, it is illegal to marry your sibling, parent, grandparent, grandchild and various other blood relatives.
The identity of the twins has not been revealed, nor their ages or where they are from. Nor is it known how they met or how they found out they are twins. Experts, however, suspect they must be at least 30 because the law on adoption changed in 1976, making it much easier for adopted children to discover their biological parents. Since then, too, every effort has been made to keep together siblings put up for adoption.
Alton raised the case in the House of Lords, not for its own sake, but to draw attention to a wider issue. He believes the risk of siblings unwittingly marrying each other is rising fast because of the prevalence of IVF (In Vitro Fertilization) treatment.
Since 1991, more than 27000 babies have been born from donated sperm, eggs and embryos. As a result there might be thousands of people who unwittingly share one parent - and who might one day meet and fall in love.
What are the risks? Do such unwitting siblings have a special attraction for each other?
And does the law need to be changed so that birth certificates always make it clear who a child's biological parents are?
The extraordinary case of the twins who married, is not the first time a brother and sister have been lovers without knowing their blood relationship. In one case in the US, a Polish couple had 13 children and gave up nine for adoption.
They were all placed with families in a fairly small geographical area. Some of the children were later told they had been adopted, others were not.
Gary Klahr grew up not knowing he was adopted and in 1979 he met and dated Micka Zeman. They had a relationship for six months - and only later found out they were brother and sister.
"My relationship with my sister is the kind of thing that could have you jumping out of the window," he said later. "But we didn't know. Thank God we didn't get married." In Britain, William and Annette Watch had their marriage annulled in 1971 after it turned out they were half-brother and half-sister. Soon after William was born, his parents separated and William was put into care. Later he went to boarding school and joined the army.
He fell in love with Annette, whom he believed to be the daughter of Leslie Ingham, a married man who had also begun a relationship with William's mother. In fact, Annette was the daughter of Ingham and William's mother - making her William's half-sister.
When a court declared their marriage incestuous, Annette said: "I dream of having children. But it's Bill I want first. We've been so happy together and just can't bear it."
With identical twins, there are powerful reasons for a special relationship - even if they have been separated. Having come from a single egg and sperm, they have the same genetic make-up, which exerts a profound influence on their lives even if they have been brought up in different circumstances.
But with fraternal twins - who come from two eggs - and ordinary siblings, fewer genes are shared and there is no obvious reason why there should be an innate powerful attraction. Yet researchers believe a peculiar syndrome does exist in cases where close relatives meet after being separated in childhood.
Glenn Wilson, a reader in personality at the Institute of Psychiatry, said: "It's now a widely recognised phenomenon that if you meet someone you have been separated from as a child, you may find you are extremely attracted to them. It's known as Genetic Sexual Attraction, but it's genetic only in that you are closely related to that person - it is not necessarily caused by strictly genetic factors.
"People who meet family members later in life have described how they feel a 'bolt of lightning'."
Ivor Lytton was one of those who experienced the thunderbolt. In 1998 Lytton, from Edinburgh, met Rita Meadows, who had grown up in South Africa, at a dinner party in Scotland. Lytton had organised the event after tracing Meadows, who had been adopted as a baby. She was his sister - but he had no idea how powerful the effect of meeting her would be.
"From the moment we met, I was smitten," he said later. "She put a smile in my heart and a spring in my step."
It nearly ruined his marriage and his life. "I didn't choose to fall in love with her, or expect to feel sexual desire. It just happened. Even in front of my wife, I made no attempt to hide my adoration."
The danger is that siblings, unaware of their relationship, may marry and have children. The similarity in their DNA poses significant health risks.
"Siblings share half their genes just like a parent and offspring," said Steve Jones, professor of genetics at the University College London.
"So the problems which could arise from having a child with your brother and sister and having one with a parent are the same. There is little data for the implications of sibling marriage, but we know that whatever the childhood death rate is in a country, it is doubled if a child is born from two cousins.
"If a child is born from two siblings, the risks are likely to increase further. Many of us have recessive genes for genetic disorders like cystic fibrosis. If a person who has one of these genes has a child with a cousin or a sibling, the chance of disability or death increases."
Alton and others are challenging the Human Fertilisation and Embryology Bill. They argue that in the age of IVF it is more important than ever for children to know their biological past.
"There needs to be more clarity in public records," said Alton. "This is to prevent incestuous relationships, but also for reasons of genetics and disease prevention".
Such is the scale of IVF now that in the US one donor recently calculated he could theoretically have fathered 4960 children after 15 years of donating sperm. "That's the top mathematical possibility," he said.
In Britain, donors are restricted to 10 offspring.
Nevertheless, it still means the risks of half-brothers and half-sisters meeting are rising fast.
Stephen Hopwood, of Stowe Family Law in Harrogate, believes the current system gives sufficient protection.
"You have to appreciate that [the case of the twins] is almost ludicrously far-fetched, and a kneejerk reaction to difficult cases always makes bad law". - The Sunday Times, London