When I found out Dad wasn't my biological father I was shocked. But worse was meeting my new half-brother… and finding I was sexually attracted to him

The moment I walked in to the bar, I felt his eyes on me. He smiled warmly and leaned in to kiss my cheek.

Tall, dark and handsome, my breath caught in my throat as I looked up at him.

All the nerves I'd felt about meeting him disappeared and instead I felt a tingling, flushed, warm sort of feeling.

As he tipped his head back and laughed at something silly I'd said, I realised with a jolt of shock that it felt very much like a teenage crush.

I couldn't possibly fancy him, could I? What on earth was wrong with me? I was sickened by my reaction. After all, this wasn't a first date but the first meeting with my recently ­discovered half-brother, Hugh. So any stirrings of attraction were completely inappropriate.

Yet as the weeks passed, I couldn't stop thinking about him and looking at pictures on his social media accounts with lustful longing.

I appreciate this is a shocking, ­disturbing admission. Attraction to a sibling is the ultimate social taboo for good reason.

But when in panic I researched online, I took comfort from the fact that I wasn't alone.

Genetic sexual attraction is a known phenomenon and can occur when close relatives are reunited after a long period of separation.

It is not uncommon in adoption cases or in this age of donor conceptions, where people might fall for a sibling unwittingly.

Apparently it can be caused by the fact that there is a basic human attraction towards those who have similar physical attributes to us.

Hugh and I do look a little alike with the same eye colour and similar ­features. Some experts believe it to be a delayed by-product of 'missed bonding'.

In some cases the power of attraction can be so strong it wreaks havoc on existing relationships.

In my own case, I didn't dare open up to my husband of ten years. What on earth would he think of me? Quite apart from the inappropriateness, we were happily married with three children and a lovely home. I prayed it would prove a passing, if highly ­distasteful, crush.

Six months earlier, Hugh's father had written to me out of the blue, informing me that my gentle, kind dad who raised me was not my ­biological father. My mother, who had died of cancer eight years before, had had an affair with him, a family friend who I couldn't recall.

Why tell me now? In his letter he said he'd wanted to get in touch after Mum died but hadn't ­contacted me before then because he was married.

It was when his third marriage broke down that he asked a mutual friend for my contact details.

At first I thought it was some sort of ­terrible joke.

My parents had had a normal, happy marriage, hadn't they? I had no idea Mum had ever been unfaithful – the revelation was overwhelming.

Everything I had believed – my whole identity – was a lie. My biological father was keen to meet me in ­person but I had to get my head around everything first.

My husband was as shocked as me and we both wondered if it was even true. I spoke to my older sister a few days later and she couldn't believe it either. How could we not have known this about our own mother?

I didn't reply to the letter from my biological father for weeks and it was months before I dared tell my dad about it. How could I possibly broach it or ask him if he knew? What if he felt differently about me and couldn't love me any more?

To this day, we've only ever had one difficult conversation about it. He had no idea Mum had been unfaithful. I saw only sadness, no anger or bitterness. He kept saying how much he loved Mum and I could see he felt betrayed.

We've always been close and he was quite a hands-on dad, reading my bedtime story, teaching me how to ride a bike and so on. This revelation didn't change any of that; if anything, the fact he wasn't my biological dad, but had unwittingly stepped up to fulfil that role and become so central to my life, made me love him all the more.

Desperate to find out more about my identity, I bought an online DNA test. It showed I had mixed heritage with some Eastern European roots and a dozen or so distant relatives I had never heard of, including Hugh and two half-sisters.

I had another brother, but as he had never done a DNA test he didn't show up. My biological dad had already told me he had spoken to his children, two from his first and two from his second marriage, before contacting me.

While they were surprised, I was welcome to get in touch with them directly.

Would they look a bit like me? Would we get on? The first two were a couple of years older than me, the second two over ten years younger. I pictured ­Walton-style family dinners around long trestle tables, putting the world to rights over wine.

I worked out my biological father must have had the affair with my mum when his eldest two were very young. I wasn't surprised that they weren't as keen to be in contact with me at first. Hugh and his sister ­Jessica were from his second marriage, born years after me, and were receptive from the outset.

I had looked them all up online before meeting them in the flesh and couldn't help but notice that Hugh and his older brother Edward were good-looking men.

They had all attended a top boarding school and seemed to have impossibly glamorous lives – parties on yachts, music festivals, etc.

I might have gone to private school but it was a day school in the East Midlands. They had clearly had a more affluent life than me and I did feel envious of their privilege. One picture showed Hugh leaning back insouciantly on a chair, looking directly at the camera. He had a sort of half smile on his lips and one eyebrow slightly raised. With his strong jaw and athletic build, he could have passed for a model. 'Gosh, he's gorgeous!' I caught myself thinking.

When I sent a screenshot to three of my closest friends, who'd been a huge support throughout the whole thing, they joked: 'Wow, he looks just your type!'

He was indeed – just the sort of charming, ex-public school boy I would have gone for in my 20s or 30s. But now I was happily ­married and those days were long gone. Or so I thought... until we met and hit it off.

The connection with Hugh was instant. Though we both admitted to being nervous, our conversation flowed easily, and despite the madness of the whole situation, he made me feel very comfortable. 'It's so great to meet you,' he said. I confided in him about just how difficult the last few months had been.

He was sympathetic, leaning over to put his hand on my arm. I couldn't deny the spark of desire that ran through me at his touch. I didn't want the night to end but we both had to be up early for work the next day so I reluctantly said goodbye. When my husband asked how it had gone, I told him: 'Fine. It was great.' I couldn't exactly admit I had felt attracted to my own brother, could I?

After that I couldn't stop ­thinking about Hugh. We followed each other on social media and I pored over pictures of him. According to experts this fascination with a newly found sibling is not uncommon.

I even found myself imagining what it would feel like to kiss him. Would it really feel that wrong? Did he feel the same way?

Thankfully, there were no such feelings with the other brother, Edward, who got in touch a few weeks later.

He was perfectly pleasant but quite aloof when I met up with him and his sister, who kept the conversation going.

Meanwhile, Edward's eyes kept glancing around the room as though he was bored. I had hoped that Hugh would come along that time, too, but unfortunately he was working.

We texted and exchanged voice notes a few times, mostly about the weirdness of the situation, but it was difficult to maintain the messages in between work and the children.

We didn't meet up again until about six months later for a ­'family' lunch with all the siblings. Seeing Hugh again that second time with the other siblings in tow was less intense.

We chatted politely but there were none of the butterflies I'd experienced the first time, which was a huge relief. A few months later, Hugh unfollowed me on ­Instagram without explanation. What had I done to annoy him? I was gutted but I have my pride so I unfollowed him, too.

In retrospect, it was a wise move. The lack of contact calmed my mind and helped me focus on real life again.

That was about a year ago, though, and I'd be lying if I said I hadn't thought about Hugh since. But it's more of a wistful wondering rather than a lustful longing.

I occasionally look at his social media feed – thankfully, he hasn't blocked me – and can see that he's with a beautiful blonde girl now.

It wasn't until after I'd met all my half-siblings that I agreed to meet my biological father. Sadly, I found him to be a difficult, defensive sort of character and an unreliable narrator.

He's changed his story about his relationship with my mother a few times. She's gone from being the love of his life to someone he had a short fling with.

More importantly, he has refused to take any sort of accountability for the damage he has caused in not coming forward for 40 years. 'It's been very ­difficult for me,' he said, expecting sympathy. In fact, I feel nothing for him. He's little more than a sperm donor in my eyes.

I'll send him a Christmas card this year and the odd text but I don't feel the need to pursue a proper father-daughter relationship with him.

The bond I hoped I might have with the other new siblings hasn't materialised either for one ­reason or another, but I've accepted that. All in all, it's made me appreciate the sister I grew up with all the more and the lovely dad who raised me.

Looking back, I'm so glad I never acted on my feelings for Hugh; I can only imagine how catastrophic that might have been. It may have been a fleeting fantasy but it had felt very real at one stage.

And I'll never not wonder if he felt the same way.

Marianne Thomas is a ­pseudonym. Names have been changed.