I don’t remember the exact moment my sister asked me to be her birthing partner. Perhaps it was just a natural assumption we made, having always gravitated towards one another in times of need. The thought of it thrilled and terrified me. As a flighty 21-year-old, it had never occurred to me that it would be a role I would need to fulfil.

My sister became pregnant at 16, when I was in my second year of a performing arts degree in Salford and she was living with our dad. It was 2006 and my life was operating on a cycle of nights out, hangovers and minute noodles, punctuated only by a sparse timetable and occasional bar work to keep my overdraft under control.

I had more or less broken ties with the small market town in West Yorkshire where I had grown up; my parents’ divorce a few years earlier meant there was no longer such a thing as a family home. Life felt wild and untethered and partying had become my personality. Through everything, my sister and I remained close: both a little hedonistic, both more than a little damaged by the breakdown of our family. A baby wasn’t something I had imagined for either of us.

I took time off so that I could be with her for the birth, but the date in mid-September that we had circled on the calendar came and went with no sign of labour. The next day, too. And the one after that. An entire week passed before my sister, balancing a bowl of Weetos on her belly, calmly told me that her contractions had begun.

The first few hours felt like standing in line for a rollercoaster. The giddy anticipation, the nerves. But as the labour progressed, the mood grew increasingly sombre. The doctors in the hospital offered her every drug and intervention going, but my sister refused them all, easing her contractions with nothing more than gas and air. She may have been younger than every other mother on that ward, but her belief in herself was unmatched; her strength was something close to supernatural.

Usher as a child with her baby sister. Photograph: Courtesy of Emily Usher

When I had learned my sister was pregnant, my immediate reaction was fear. I worried about what people would say, how they would treat her. It summed me up. I had always been overly concerned with the opinions of others, changing like a chameleon to fit in. I envied my sister’s authenticity, her ability to move through the world unimpeded by what others thought. But, as I watched her pace, rock and roil through the increasing intensity of her contractions, I felt overwhelmed with pride. Throughout her pregnancy, I had seen how the world looked down on her; I seethed at the way she was spoken to, the attitudes that oscillated from patronising to dismissive. Never once did she kowtow to anyone’s judgments. Always she held her head high, rising above whatever was thrown at her.

Finally, at 10.18pm that night, I watched in awe, shock and utter incomprehension as my baby sister brought my indescribably perfect baby nephew into the world, all by herself. In the preceding months, she had taught me so much about resilience, self-reliance and strength. But witnessing the raw and bloody miracle of a new life changed my perspective in a way I couldn’t have imagined. The wonder of our existence, how utterly bonkers it is that any of us are even here, hit me like a thunderclap.

Holding my nephew in my arms a little later, I felt an immediate rush of love. How strange it was to look into his tiny face and see my sister, mum, dad, siblings. Myself. His arrival pieced us back together, albeit in a different form.

When I returned to university a week or so later, something in me had shifted. Seeing my little sister change from a carefree girl into a mother brought the fleetingness of time into sharp focus. Suddenly, every day felt valuable, the opinions of others less so. Instead of wasted ramblings with strangers at parties, I craved fulfilment in my interactions. I started to focus on my degree and took up a placement teaching creative arts in a female prison.

My sister moved into her own place, a little terrace on the same street as our primary school. Spending time with my nephew became a priority. I realised I wanted to be someone he could look up to, someone he would be proud of.

I had failed my first year of university, but I graduated the year after my nephew was born with a first-class honours degree, my sense of self stronger than it had ever been. Since then, I have experienced the same sensation of astonishment and grounding with the arrival of my own three children, with the memory of my 16-year-old sister’s transcendent strength propelling me through each of their births.

My nephew is studying for his A-levels now, a bright future ahead of him, while my sister, who has raised two incredible sons, has completed a law degree. So many times over the past 18 years, I have wondered how my life could have turned out had I not been there to watch my nephew crown into the world, where the reckless path I was following might have led me. Each time, I am reminded of what I learned that night: the mad magic of life and the importance of making our time here count.

Wild Ground by Emily Usher is out now (Serpent’s Tail, (£16.99). To support the Guardian and the Observer, order your copy at guardianbookshop.com. Delivery charges may apply

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