It is so hot I could melt. I am sauntering along the genteel pavements of Henley-on-Thames, on the family Sunday of the famous festival. The air is thick with whoops and shrieks: a conjuror’s tricks are blowing two dozen well-schooled little minds on a lawn nearby. A band plays; stilt-walkers stride among us. We stroll along: a Jamaican-Nigerian woman with braids down her back, a bearded Englishman and two white, primary-age children with ice-creams. Could I be the nanny? No, I am holding hands with the bearded bloke. Do I imagine that one well-heeled local raises her eyebrow in evident displeasure, as if we are the most unexpected spectacle of the weekend? Wounded, I smile broadly, and reach for the wet wipes in my handbag that have so recently replaced the Red Bull and B&H. I am a stepmother.

How did I get here? In 2003, I met Peter, my husband, at the marketing company he ran. He was a strung-out but devoted single dad to five-year-olds, who had been separated for several months. He was 42 and was, for all those reasons, decidedly off my radar. I, aged 29, blithely jacked in my job and skipped off to Cuba alone to enjoy a month’s sub-Hemingway adventure on my credit card. When I returned, I had two realisations: one, I needed my old job back and two, my former boss seemed exceptionally pleased to see me. A lunch meeting led to a dinner and then, to my astonishment, we were dating. Fast-forward a few years and I am rocking up to his house in Oxfordshire with my suitcases, sick with nerves about the first school run. One moment I was a single woman, the next I was raising children – who were conspicuously “not mine”.

Although we would not marry for some years, I was de facto stepmother from virtually day one. And has there ever been such a maligned family member? She first enters our consciousness in fairytales: wicked seducer of naive fathers, enemy of put-upon children. How much more twisted would that tale be when the stepmother was an African-Caribbean woman who had landed in the middle of a quintessentially white, middle-class childhood? But true tales are rarely obvious: they can be joyful, sometimes painful, tender, full of twists and, if you’re lucky, quite brilliant.

The twins were eight when I moved in and I can still picture their faces as we broke the news that Daddy’s friend would be living with them. I saw in the way that they tried to keep their expressions blank that their growing affection for me was wrestling with a tumult of emotions. As for me, I was overjoyed, and petrified. I did not know how to be a stepmother; I had never had one. The central tragedy of my childhood occurred when my father, a devout, rock-solid, Hertfordshire GP, had a breakdown and left the family. The devastation that this wreaked on my family ripples on to this day. Such was my ache for our family that was, that I did – and still do – over-identify with the offspring of divorcees. And I know this: if my own father had tried to get together with a woman other than my mother, I would probably have given the poor woman hell.

So, needless to say, I was clearly not cut out for this stepmother lark.

Psychologists tell us that we parent according to the example we are given. I had been raised by a big-laughing, love-them-all-the-way NHS nurse of a Jamaican mother. So, as I could not stepmother them, I mothered them. Over the ensuing 15 years I would do that once-daunting school run and everything else a thousand times. Yet I would never be so insensitive as to insist that I was their mother. No: I haphazardly undertook to love them as if I had pushed them out of my own womb.

At first, Emma and Charlie accepted that I was in their life, but naturally they pushed back at times. This came out in the smallest, yet oddly painful, acts of resistance – sharply chiding me for not sticking a birthday card to a present – “No, no, that’s wrong. This is how we do it here.”

Emma’s hair was another source of terror, early on. It was golden and beautiful, truly her crowning glory. Beyond the brush, wash, condition routine, I had no idea how to care for it – mine was a world of afro hair, all plaits and relaxers. All I can say is, thank God for YouTube.

Managing Charlie was easier as he and his father shared a passion for sport, especially cricket. I just had to keep replacing the mislaid kit and his dad would do a lot of the rest.

However, in stepfamilies, insecurities can emerge on all sides. Young children have a right – a need – to be able to take their parents for granted and they feel they cannot do that with a step-parent, not for a long time. Children of broken relationships can be anxious and, even if you approach them with kindness and understanding, they may find your very existence to be a source of worry. I know that Emma and Charlie worried I would not like them. They worried I might not ever love them like a real mother. They worried I might take over their lives and, I sensed, that I might one day leave them. As we share neither genes nor race, I believe they also worried that I couldn’t truly understand them (those girly hair conversations became more layered) and, although they hid it from me, I know that, as with all kids, they went through a stage of worrying what their school friends thought.

The great panacea, though, is laughter. My stepson makes me roar, especially when I shouldn’t. Of course, teenage boys are rarely the most PC and I once had to explain why the Mexican jokes sweeping his class were neither funny nor to be taken literally. Children can also alarm you – my young stepdaughter once joked that I was “quite white”. She did not mean to insult and she is now socio-culturally very switched on, but it shook me. What life lesson had I failed to give? Did I need to clarify that education or broad cultural references did not make one white, that I would always and ever be black, and that white privilege could not be married into or absorbed by osmosis, nor was that an aspiration? No: I just needed to chill the hell out.

We did talk about race, although far less often than many would suspect. When they were very young and anxious in ways they did not yet understand, they would ask me why I was black. I would explain about melanin and Africa, the usual. Then, one time, I did something I would wholeheartedly condemn in a normal adult conversation – I told them I was made of chocolate. They giggled: thrilled, amazed, unsure. I insisted, “taste my arm”. This might seem like a total no-no in terms of correct racial awareness, but hell, they were only five and wary and confused and I needed to tell them I was sweet and would be kind, and if the Easter Bunny can deliver you a stepmother, maybe life is not so bad.

If my deepest fear was that it would all go wrong and I would be left with prejudiced children who did not love me, then my writer’s instincts saw me through. Show, don’t tell. Don’t lecture about racial harmony, just live it: love them. Don’t try to explain that we are all equal and the same: be yourself, and love them. Today, I love the fact that it is Charlie – my exceptional, empathetic stepson studying law at university – who lectures me about grime.

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I love my stepdaughter. The truth is that the stepmother/stepdaughter relationship can be particularly fraught and heartbreaking. After all, you both love the same man, you are sharing one home, perhaps vying for the upper hand, especially during the teenage years. Moreover, at the core of your relationship lies one stark fact – you are a woman who is not her mother who is with her father. Hard. However, the relationship can also be beautiful. Emma made me weep with her schoolgirl singing. I love that her attitude went from mid-teen “meh” to studying politics. She is my barometer for measuring what matters.

So, fast-forward once more from the school days. We are walking along a Caribbean beach: so hot, I could melt. We are a motley crew, the black woman with the braids, the white bearded man and two white teenagers, one boy and one girl.

A lone Rasta sits on a large chunk of driftwood, watching us go by. He waves me over and I slow to chat. “You from England?” he asks, doubtfully. “Who are they, you with them?”

“Yes, man,” I reply, smiling. “They’re my kids.”

• Darling, Rachel Edwards’ debut novel, is published next week by Fourth Estate.