"Why I couldn't look away from Cate Campbell's face at the Commonwealth Games." 

When Bronte Campbell, 23, touched the wall with her fingertips just half a second before Canada’s Taylor Ruck, spectators at the Gold Coast Aquatic Centre roared with excitement at yet another Australian victory.

But it didn’t take anyone long to see that what appeared to be an extraordinary win, had hidden within it a story of loss.

On the sidelines stood Cate Campbell, her hair dry, in her green and yellow Commonwealth Games jacket. Her hands applauded, but her face betrayed a sense of defeat.

The night before, the 25-year-old was meant to win the 100m freestyle final.

That was until her younger sister smashed her own personal best to win Gold, leaving Cate standing a few inches below her on the podium, a silver medal around her neck.

An hour before the women's medley final, it was announced Bronte Campbell would represent Australia. Cate would watch.

She watched as Bronte won a Gold medal that could have been hers.

Perhaps I am reading into Cate's face a story that is mine. Because what I saw, at the moment her sister achieved something she would have liked to, was an expression I deeply recognised.

It wasn't jealousy - that's far too much of a simplification.

When you are close to a sibling who excels in the very thing you excel at, whose identity hinges on the very same thing your identity hinges on, then there is no such thing as first and second.

There is simply a winner and a loser.

All her training has transformed her into one of the best swimmers Australia has ever seen. She didn't come second at the school swimming carnival, she came second at the Commonwealth Games.

But achievement is not that simple between sisters, especially so close in age.

I do not know life without my twin sister; the person who I love more than anyone in the world, but who is also my greatest rival.

The dynamic has its advantages. We are intensely competitive, towards one another but also towards the world more broadly. You can see how hard the other is working. You can see, up close, where their strengths are.

But it comes with a deeply complex set of emotions that I'm not sure we quite have the words for.

Cate's face, from my perspective, did not convey a hint of envy or disdain. It was a look of discomfort, as though she was muddled by having so many competing thoughts at once.

Of course she was excited for her little sister, an achievement she will be proud of until the day she dies. There was sadness. Self-flagellation. A hint of disappointment. A determination to do better.

We discuss on Mamamia Out Loud why the relationship between siblings can be so complicated. Post continues below.

But I think there might have been another woman, in the same room, having an equally complicated response to a moment that should have elicited nothing but bliss.

Because that moment would have been just as hard for Bronte.

When you're a set - twins, brothers, sisters, or perhaps even best friends - victory is never simple.

Your win means someone lost. The spotlight inevitably casts the person closest to you in the shadow. And part of you, at least from my experience, is always a little bit sorry.

It's a logical fallacy, and I'm not sure it's particularly healthy. But it's true.

Cate's ultimate rival isn't flying back to Canada or the UK.

She's likely returning to the very same dinner table.

And that must be bloody hard.