Perhaps it is because Danielle Heaney and Nick Cameron don't look in the least alike that they pass so easily for a pair of young lovers no different from any other.
She is a petite, delicate, blue-eyed blonde, while he is a strapping young man with auburn hair and soft brown eyes which never stray for long from her face.
Forbidden love: Nick Cameron and his half-sister Danielle Heaney are hopelessly in love. A judge says if they have sex, they'll be jailed.
They hold hands, they kiss, they stroke each other's arms, they listen attentively to each other. They are totally besotted.
What makes this scene so disturbing, however, is the fact that Danielle and Nick are half brother and sister.
Incest remains one of society's last taboos, as this troubled young couple know only too well. Prison is the ever present threat to this forbidden union.
"I know that loving my brother in this way is wrong morally and legally, but it just feels right," says 22-year-old Danielle.
"The only way to explain it is to say that the day I met Nick, I felt I had finally met my soulmate. Everything clicked. I would marry him if I could."
Nick first met Danielle when she was 20 and immediately thought: 'Wow, she's attractive'
Nick, 28, adds: "My feelings are very confused. We are very deeply in love with each other, but sometimes I think 'she's my little sister. I shouldn't be feeling this way.'
"All I know is that for the first time in my life I feel I belong. I should feel ashamed of it, but I don't."
Danielle and Nick have different fathers but the same mother. They grew up apart after Nick was placed in foster care as a child, and only met as adults in August 2006.
At the meeting in their mother Susan's home in Glenrothes, Fife, they were both struck with an unexpected thunderbolt of recognition, physical attraction and almost instant longing.
Within three weeks they were lovers and Danielle's marriage to her 28-year-old husband was over.
He moved out of the marital home with their child and it is he who now looks after their four-year-old daughter.
As for their mother Susan, 48, she rues the day she invited her lost-long son back into the family hoping to make up for all the years she missed with him.
It was Susan who reported them to police after walking in on them making love in the autumn of 2006, and shouted in horror: "What you are doing is morally wrong."
She is barely on speaking terms with them now.
Earlier this month the pair were put on a year's probation by Kirkcaldy Sheriff Court after admitting incest, at an earlier court hearing.
Danielle says she and Nick 'clicked straight away'
They were warned that if they had sexual intercourse again they could face a jail sentence of up to two years.
After nine months enforced separation, which was part of their bail conditions, the pair celebrated their reunion with champagne and Nick was this week planning to move back into Danielle's flat in Glenrothes.
Surely they are playing with fire? Both insist they will not have sex again, but admit they are still infatuated with each other. Both still act like lovers rather than siblings.
They've even discussed moving to another country, such as France, where incest is not illegal so they can live together as partners - although they would not be allowed to marry.
They accept that, even if they were to remain lovers, they can never have children together because of the genetic risks to the child of inbreeding.
"Those nine months on bail when I couldn't see or speak to Danielle were sheer torture," says Nick, a croupier, who moved to Glasgow to live with an uncle after their arrest, "I went through a profound depression.
"It felt to me as if we'd only just found each other and here we were being separated again. We just want that closeness back. We don't want to be apart.
"We have an unbreakable bond. Of course, it will be hard living together and there will always be temptation, but we have decided that we can still love each other without having sex.
"I love Danielle and because of that I don't want to do anything which might put her in prison, and she feels the same way about me. The sexual expression of our feelings is only one part of the relationship."
Danielle, a former hairdresser, adds: "What we can't live without is the closeness and intimacy. We can still talk, we can still go for walks, we can still love each other.
"Legally we can still kiss, still hold hands, still carry on together, we just can't have intercourse."
Short of installing a CCTV camera in their home to ensure no law is broken again, we shall just have to take their word for it.
The pair say they plan to go to joint counselling to try to understand their intense and confusing feelings.
"Obviously there have to be boundaries, because incest is illegal in this country," says Nick, "but maybe, with counselling, we can move our relationship on to a more normal brother-sister one.
"That may not be what we want at the moment because we are in love, but perhaps those feelings will fizzle out and we will be able to keep the bond that we have within the proper boundaries."
While Danielle and Nick's story is undoubtedly shocking, it is not as uncommon as many of us might wish to think.
Genetic sexual attraction is a recognised psychological phenomenon, which sometimes affects siblings or blood relatives separated at birth, who then meet later as adults.
The term is believed to have first been coined in America in the 1980s by a woman called Barbara Gonyo, who wrote about the unexpected lust she felt for the adult son she'd given up for adoption 26 years earlier.
The relationship was never consummated because those feelings were never reciprocated and they eventually faded when her son married.
According to research, first published in the British Medical Journal in 1995, by Dr Maurice Greenberg and Professor Roland Littlewood, 50 per cent of people seeking post-adoption counselling "experienced strong sexual feelings in reunions" with their real family.
This can happen between siblings, mother/son and father/daughter and is believed to be the adult response to the absence of "bonding" in childhood.
The natural repulsion brothers and sisters often feel for each other as children is a safeguard against incest and those who miss out on that bonding, according to psychologists, can develop obsessive feelings for their sibling as an adult.
Those feelings may or may not become sexual, but those that do take that course challenge our notion of incest because there is no coercion or abuse between consenting adults.
Danielle and Nick believe that they too are victims of genetic sexual attraction and are only speaking now because they want to highlight an issue few people are prepared to talk publicly about.
Either that, or they are trying to find psychological excuses for behaviour that many would consider reprehensible.
They may not have been able to control their feelings but it was their choice to act on them, ignoring their responsibilities not only to each other, but their family.
"Our mother and my foster family would much prefer we kept quiet," says Nick, "but this isn't just about us. There are plenty of other people going through exactly the same emotions. There needs to be more understanding."
Nick was just a one-year-old when his parents' relationship broke up, and he has never had any contact with his biological father.
He was placed in foster care when his mother Susan found it impossible to cope on her own.
He had sporadic contact with his mother over the years, but only met Danielle - the product of Susan's next shortlived relationship - once when she was five years old and he was 11.
Nick says he was happy growing up with his foster family in Glasgow, and eventually went on to take a degree in music and drama.
"When my mother sent a message through my foster mum two years ago to say she wanted to meet me, I initially thought: 'What does she want?'," says Nick.
"I was curious more than anything, and agreed to go and stay with her for a couple of weeks. I think she had regrets about putting me in foster care and wanted to make amends.
"I remember it feeling very strange when she collected me from the bus station and gave me a cuddle, but when she took me home I had a real feeling of belonging.
"Danielle was waiting back at home - she was 20 then - and the first time I saw her I thought: 'Wow, she's attractive,' but then I pulled myself together telling myself 'That's your sister you are talking about'."
Danielle adds: "I was nervous about meeting Nick because although he was my brother, he was also a stranger, but when he walked in I gave him a cuddle because my mum had, too.
"We just clicked straight away. It's impossible to explain. I just felt drawn to him, as if he was the person I'd been waiting for all my life."
During those first two weeks, Danielle and Nick spent every spare second together and initially there was something childlike in their touchy-feely horseplay - except that they weren't children any more.
"I think Mum could see what was happening because we were flirting quite a lot," says Danielle.
"We just felt this need to keep hugging each other and mum would say: 'Will you put him down.'
"But at that stage nothing had happened between us and I really didn't think anything would - but then one thing led to another."
This despite Danielle being married and having a young child. In fact, many people will feel her betrayal of her husband as equally as unpalatable as their incestuous relationship.
Danielle, whose own father split up from Susan when she was a baby, married at 16 in her quest to create her own more stable family unit.
She was 18 when her daughter was born, but suffered post-natal depression and found it difficult to bond with her child.
Her marriage - her partner is a house husband - was under severe strain by the time her long-lost brother turned up.
Nick says: "I felt more guilty about the fact I was sleeping with a married woman than I was about sleeping with my half-sister.
"I kept saying to Danielle 'are you sure about this? Are we doing the right thing?' because she had a husband and a child.
"She told me her marriage was already in trouble when I came along, but that is the one thing I do regret. I didn't want to hurt anyone else, but it felt as if we just couldn't stop ourselves falling in love."
With relations with his mother increasingly fraught, because she was deeply suspicious of the blossoming closeness between her children, Danielle invited Nick to move in with her and her husband and child.
Still, she insists she has no regrets or real feelings of guilt. "No I don't feel bad about it," says Danielle defiantly.
"Nick did not break up my marriage. It was already in trouble.
"Nick and my mum weren't getting on too well, so I thought the best thing would be if he moved in with us. He was my brother."
Then one day, while her husband was out with their daughter, Danielle and Nick found themselves kissing, and before they knew it they were making love on the sofa.
"My mum came round to give me some mail which had been delivered to her house," says Nick.
"She knocked on the door but when there was no answer she walked in. We'd forgotten to lock the front door.
"When she saw us she shouted at us and then stormed out. Then she came back and kept saying: 'It's wrong, you shouldn't be doing this.'
Danielle adds: "When my husband came back, I knew I had to tell him before my mum did. I said 'I don't want to hurt you, I never wanted to hurt you, but I'm in love with someone else.'
"I didn't have to tell him who. He had already guessed. He was almost eerily calm about it.
He didn't shout or make a scene. He just told me eventually that if it never happened again, perhaps he could forgive me and take me back for the sake of our child."
But Danielle didn't want him back. Ignoring her marriage vows, the needs of her child, the disapproval of her family and the law, she decided she wanted her half-brother instead.
Today, she says she sees her daughter whenever she wants and that relations with her husband, whom she is divorcing, are amicable.
"When my daughter was born, I suffered so badly with post-natal depression that I found it difficult to bond with her," she says.
"I still don't feel we've bonded properly. When my husband moved out, he wanted to take her and I didn't object - perhaps I was in no position to do so. I see her every day.
"She's too young to understand what's happened, and I don't know how I'll explain it to her when she grows up."
Her feelings for Nick appear to be so all-consuming that they have obliterated every other consideration.
Nick says: "Our mother feels very strongly that what we are doing is wrong both morally and legally.
"My view is that we are trying to deal with a new and unnatural situation. I don't see things in terms of right and wrong, I see the circumstances surrounding it.
"We both know that had we grown up together we would have had a normal brother-sister relationship, but we didn't.
"People can say 'that shouldn't happen' all they like, but it did and we want to understand the reasons why, because I do feel very confused.
"Sometimes I feel as if Danielle is my partner, other times that she's my sister, other times almost as if she is a mother to me.
"So many times I have thought 'Surely, this can't be happening. What's going on here?' Until I met Danielle, I never knew what it was like to be so swept out of control by my emotions.
"She is my closest family and my natural family so it feels right being with her. Then sometimes I feel divided in myself because she is my sister."
Danielle adds: "Mum is always saying that she wishes she'd never invited Nick back into our lives. She thinks it was a huge mistake, but I don't.
"I'm really pleased she did. If he'd been in our lives sooner perhaps this wouldn't have happened. We knew what we were doing was against the law, though we never lied about it, and we were terrified of going to prison.
"But now that we have been banned from sleeping together, we are having to acknowledge that the sexual side of the relationship is over and that we can still live together without breaking the bond we have.
"People may be disgusted by what we have done. People may say we are morally wrong and should have stayed away from each other, but we don't want to. I can't imagine life without Nick now."
Scarlett Johansson attends The A Funny Thing Happened on the Way to Cure Parkinson's benefit for the Michael J. Fox Foundation. (Photo by Stephen Lovekin/Getty Images North America) 
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