In the UK a pair of twins who were adopted by separate families as babies got married without knowing they were brother and sister.
They met later in life and felt an inevitable attraction. " Everything was great at first." said the unnamed couple, " It was like we could read each other's minds and the sex was intuitive and natural.
Then we started to bicker a lot which later got physical with hair pulling and flicking the backs of ears, we soon got suspicious after a few years when our first three children were born mongs."
Mo O'Reilly, director of child placement for the British Association for Adoption and Fostering, said the situation was traumatic for the people involved, but incredibly rare, it makes me feel dirty just thinking about it.
"Thirty or 40 years ago it would have been more likely that twins be separated and, brought up without knowledge of each other, in fact I'm my own parent's sister ."
"This sad sick case illustrates why, over the last 20-30 years, the shift to openness in adoption was so important," Ms O'Reilly added.
Pam Hodgkins, chief executive officer of the charity Adults Affected by Adoption (NORCAP) said there had been previous cases of separated siblings being attracted to each other.
"We have a resistance, a very strong incest taboo where we are aware that someone is a biological relative though it is generally accepted that all males have sexual fantasies about their mothers and sisters," she said.
"But when we are unaware of that relationship, we are naturally drawn to people who are quite similar to ourselves then you have the Brangelina syndrome."
A court annulled the British couple's union after they discovered their true relationship. The couple are now trying to put back together their lives as brother and sister and are trying for another child.