At the World Championships in Kazan, there was a smile on Australian swimmer Cate Campbell's face right after she had relinquished her 100m freestyle title, finishing third.

Athletes rarely revel in defeat - except it was her younger sister Bronte who took her title. It was one of the biggest occasions where Bronte, 21, upstaged Cate, 23. They hugged in the water, long after others had exited the pool.

Asked if there was any sibling rivalry between the two, Bronte, who with Cate is in town for the Singapore leg of the Fina Swimming World Cup, said: "It's not really that pronounced. Since we were little kids we've always had this dream to (represent Australia) together. So when you're on the world stage, it's sort of like, together you're going up against the world, it's not really against each other as much."

One gets the sense that it was an honest reply. The two did exchange awkward glances when asked about sibling rivalry, but a smile was all it took to diffuse the tension.

Cate added: "It's like Campbells against the world."

Siblings going head to head is not a new theme in sport. Tennis has the Williams sisters, basketball the Gasols, American football the Manning brothers.

But Venus and Serena have different hitting partners and entourages, while the Gasols and Mannings compete in team sports.

Things are different with swimmers, and more so in the Campbells' case.

They live in an apartment they co-paid for and train together six days a week under the same coach in Brisbane. But they also compete in the same events, and at next year's Olympics in Rio de Janeiro, will probably be each other's greatest rivals for Olympic gold.

After all, the sisters are top two in the world in the 50m and 100m free this year - Cate is fastest over one lap, while Bronte's 100m time in Kazan was a world best this year.

Their close ties were easily reflected during the interview at Swissotel The Stamford hotel. They finished each other's sentences, teased each other, and gushed when talking about hometown National Rugby League club Brisbane Broncos or J.K. Rowling's Harry Potter novels.

Cate said: "It's really good to have a sibling in the national team. We've both always wanted to go to the Olympics, and it's so much more fun going with someone you like."

She quipped: "I do actually like her, you know."

But the relationship between the two was not always so smooth. Bronte, the harder worker, was quicker when they were younger. Once, she flaunted her gold medals by wearing them around her neck at home. Cate took them and hid the stash under her bed.

With age comes maturity and the realisation that there is more to life than competing. They also have three other siblings who are not competitive swimmers, because, Cate jokes, "they look at us and saw how much hard work it takes and they're like, 'I'd rather do something else'."

Their upbringing also helped. Born in Malawi, Bronte recalls seeing people "living in mud huts and fighting for food", adding: "This helps us appreciate what we have."

With the Olympics on the horizon, observers are wondering if they can join French swimmers Laure and Florent Manaudou as siblings who have won Olympic gold in swimming. Cate already has a 4x100m gold from the 2012 edition. Said Bronte: "That would be a dream, but it's going to take a lot of work to get there. You can't really make a prediction about the Olympics."

You cannot, but as they keep pushing each other in the pool, odds are, both sisters will be smiling in Rio.

Chua Siang Yee