My relationship with my big ­sister is one of the great joys of my life. We share a love of running, yoga and ­literature. We are both ­thoughtful, caring and have a tendency to get lost when navigating public ­transport. But our love comes wrapped in pain – through no fault of our own.

We studiously avoid certain topics of ­conversation. And I catch her slipping into another room and shutting the door if her mother calls when we’re together. I’m one of those – ­whisper it – ‘children of an affair’, a lasting reminder that ­sometimes a ‘mistake’ can become a lifelong legacy.

My sister was 12 when I was born and her mother was – and still is – married to our father. For over 30 years, he kept them in the dark about my very existence.

My sister and I connected only a decade ago when I was 32; my mum had died nine years before. My sister couldn’t have been more welcoming but, however close we’ve become, there’s ­something that can’t be denied. All these years later, I still feel like a dirty secret. And it still hurts.

Although I’m now a 42-year-old, happily married mother of four, I still feel like the ‘other’ child. The one that doesn’t get to be in family photos, isn’t invited to family ­parties or reunions. Our love is like scar ­tissue, the wound caused by my father’s betrayal always close to the surface.

I was reminded of all this when the Foo Fighters frontman Dave Grohl revealed last week that he had fathered a child ‘born outside of my marriage’. Some praised him for apologising, accepting responsibility and vowing to do right by his wife and kids, including being a ‘loving and supportive parent’ to his new daughter.

Only time will tell if he stays true to his word, but that phrase ‘outside of my ­marriage’ sent a shiver down my spine. A sense that the child is already relegated to the outskirts.

I can’t help feeling that Grohl’s ‘other’ daughter will feel the consequences of his lies and deception for years to come, no ­matter how loved she is, or how rich and famous her father happens to be.

The stigma of illegitimacy is long-lasting. Believe it or not, there are those who steer their husbands away from me as if I’m carrying a contagious infidelity gene. (The irony being that I’m the most devoted wife ever.)

Then there’s the pain and guilt from knowing your existence is the root of another family’s ­heartbreak. Your siblings’ ­heartbreak, no less.

I’ve been navigating these issues for years, and I don’t pretend to have all the answers. But I can now speak aloud about the shame I’ve been conditioned to hide. That feels like progress, but it is hard won.

I now live in London, but I grew up in Eighties Manhattan. I was a rarity for the time: an only child of a single immigrant mother from Ukraine.

Having a parent with a ­pronounced ­Eastern European accent and no dad to speak of already made me stand out in all the wrong ways. I was lucky my mother loved me with the fervour of an entire village. And, during my early years, I didn’t really ­consider or care about who my own father might be.

But I did want to fit in, so ­sometimes I made one up. Then I’d forget his ‘name’ and get caught out. I remember one child saying, ‘My mum says you’re a liar! You don’t have a dad and you can’t come over for a playdate because your mum might try to steal my dad.’

My Mum was ­constantly gossiped about, but she never let people see how much their words wounded her. The fact I felt so safe, blanketed in my mother’s adoration, meant I didn’t ask her the questions I now wish I had asked when I’d had the chance.

I know my parents had worked in the same industry, met at an international conference and had a fling. That, at 35, she ended up pregnant and alone. But I’m not sure if their ‘relationship’ was confined to one weekend or whether it lasted months or even years. I’m not even clear whether my mum knew my father was married with two children, aged 12 and ten when I was born.

I would love to know her side of the story. What drew her to my father – was it love? Or a stupid drunken tryst?

Her love for me was so intense that she reached out to my father only when I was 13 and she began to worry about her own health and ability to provide for me. Though her health concerns were described to me as physical ­ailments, I now understand that this was the first of many mental health crises she would have in my lifetime.

Mum was spontaneous and zany. She wore bat capes on the school run and sent me to ballet camp in Monte Carlo summer after summer. So when she booked a last-minute ‘European adventure’ for us in December 1995, it wasn’t out of character. Neither was her secrecy: she didn’t mention she was taking me to see my father to try to persuade him to take a DNA test. She told me the man we were ­visiting was a former colleague, nothing more.

In a strange turn of events, I became extremely unwell from the moment I got off the plane. I spent most of that week in bed with ­norovirus – delirious, vomiting and sleeping the days away.

Years later, I found out from my sister (and a picture that I spookily found one day and lost the next) that she and I had met on that visit. Neither of us knew what we were to one another.

I’d been so unwell, I didn’t ­remember meeting her. Now I ­wonder if this was some kind of stress response. My body’s way of processing something unknown but momentous.

Several months later, back in New York, my ­mother’s ­‘former colleague’ was in town for work and took me shopping at ­luxury department store Barneys. ­Without my mum. The experience was surreal, almost like a scene out of a teen movie.

Kind stranger buys gawky teen designer cashmere; she ­blossoms from duckling to swan. Except there was no blossoming, and the kind stranger disappeared abruptly. I wouldn’t hear from him for another decade.

I was 14 at this point and though I suspected who this man must be, it didn’t make any sense. If he was my father, wouldn’t he want me in his life? Though I hadn’t agonised about it earlier, his appearance, and rapid disappearance, resulted in a confusing ­jumble of emotions.

Instead of trying to untangle them, I found solace in fashion. I wore the black, ribbed Calvin Klein cardigan with brown stripes and front zip that my dad had bought me nearly every single day for the next year. It connected me to something, to someone, beyond the life I knew, and that made it special.

In my latter teenage years, my mother’s mental health started ­teetering again. She became increasingly panicky, anxious and reliant on the tranquiliser Xanax. Her behaviour, more dramatic and unpredictable.

It was the first time I started feeling scared, for both of us. The first time I wished there were other adults around to confide in, to help me.

It was at this point that my ­mother decided to establish paternity once and for all by going over my dad’s head straight to the legal system. He couldn’t refuse a court-ordered DNA test.

On my 21st birthday – by then at university in England – it wasn’t the cheap booze making me feel bubbly inside. I finally had the proof – a 99.999 per cent DNA match – that my father was my father.

I was happy to know who I belonged to, even if it didn’t change much in terms of our ­day-to-day relationship (i.e. we had none). My father did start to ­communicate with my bank, at least. Other than that cashmere spree, he had never paid a penny for me. My mother had never asked, nor received, financial ­support from him.

I didn’t get to celebrate for long. The following year, my mother had a psychotic breakdown. She suffered months of delusions and hallucinations, losing her grip on reality day by day until, tragically, she took her own life.

That haunting final year continues to devastate me to this day.

Just as she and I were finally starting to connect as two adults, when we’d reached the point where we could have frank and meaningful conversations, she was gone.

It was my mother’s death that pushed me to try and bond more deeply with my father. In the space of months, I’d transformed from a sparky 20-something filled with hope and optimism about my future, to someone unrecognisable.

I’d practically become an orphan overnight, and since my mother had sold our apartment in Manhattan to help pay her debts, I found myself with nowhere to live and no ­financial security. It was terrifying, and I needed a parent desperately. I phoned my father and he finally agreed to meet me in the lobby of a hotel in London’s Park Lane. Ever the optimist, I naively pinned all of my fantasies on a total stranger.

It was a huge letdown. Jokes were made at my mother’s expense. I didn’t know how to respond, so I kept smiling.

I did enjoy hearing stories about his two children, a girl and a boy. A kernel of hope allowed me to imagine I would be ­gaining new family members after losing mine. They wouldn’t replace my mother, but I’d take any ­consolation prize I could get.

I look back on that grief-stricken young girl now with pity, frustration and rage. This man was extending an olive branch when I needed a tree, a garden, the whole house.

While this pas de deux went on for the next decade, our ­relationship took on the rhythm of that first shopping trip. We’d meet once or twice a year and since he travelled frequently for work it was always in one city or another: New York, Washington D.C, Paris, London.

Meanwhile, I spent my 20s ­trying to repair the harms of my childhood by drafting a new, happy family story. Not that I realised it at the time. I moved to London at 25, bought a house, a bulldog, got married and had my first baby at 28.

I found fulfilment in those early, milky days of motherhood. And in my writing: I’d started working as a freelance journalist and copywriter.

I was 32, happily married and expecting my third child by the time my half-siblings discovered I existed. Of course I had looked them up before we got to this point. I’d even saved their contact details during my many sleepless nights feeding babies, but I never dared press ‘send’ on any emails.

The idea of crushing their lives and potentially destroying their ­relationship with their father felt selfish and cruel.

He even told me that he planned to finally tell them the truth; I simply didn’t believe him. Not until the first email arrived from my sister.

It was a gorgeous ­message, telling me she’d just found out, and providing me a bit of a ­biography because, of course, we were strangers. She ended with an invitation to continue the conversation, which we did.

Then, after a lengthy email exchange, she showed up at my door in London.

She embraced me and her hug said it all. It wasn’t my fault. We were in this together. We could move forward together. It was the most generous thing ­anyone’s ever done for me.

I don’t know exactly how my father told his wife and my half-siblings about me; I’ve never dared to ask for the details, and I hate the thought of my existence causing pain to anyone. I wept for most of that ­weekend. Tears of sadness but also hope. My sister and I talked for hours, asked three decades’ worth of questions. We tried to piece together our life stories from the wreckage of our father’s (and my mother’s) lies.

We have many similar features – eyes, cheekbones – and lots in common. She’s also married with children, and we have ­identical wedding bands that are far from a typical choice. Seeing that same ring on her ­finger reassured me that our connection would be powerful.

We are close friends to this day. We go on holidays together with our families. We’ve created ­something beautiful out of the mess. I have yet to meet my half-brother, but I did receive a ­heartfelt email from him.

As for my father, I kept trying to make our relationship work. I could tell he felt relieved that I’d hit it off with my sister. He told me she’d ‘look after me’ but he missed the point: I only ever wanted my daddy.

Finally, two years ago, I decided to be brave. Not by saying ­goodbye to a real relationship, by acknowledging the death of a long-held fantasy. I stopped picking up the phone when he called. I quit asking my sister about him.

I’m left with many questions I’ll never get answers to.

So many secrets remain in the dark. But by focusing on those who love me unconditionally – my husband and our four ­wonderful children – I’m running towards the light. Further from the shame of being the family outcast.