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While some siblings share their mother's eyes or a sprinkling of freckles across the bridge of their noses, others share the same incredible athletic ability. There are 36 sets of siblings competing in Rio. Some are working together, like the Williams sisters; while others are competing head-to-head, like Estonia's Luik triplets.
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Here are the siblings who are making the Rio Olympics a family affair.
1. Serena and Venus Williams
Arguably the most famous siblings to grace the Olympics, Serena and Venus Williams, three-time gold medal winners for doubles tennis, exited the Games early when they fell to the Czech doubles team. It's their first Olympic loss after winning gold in Sydney in 2000, Beijing in 2008 and London 2012. Both were eliminated in singles competition, too, but Venus picked up silver in the mixed doubles.
2. The Luik triplets
To prepare for Rio, Estonian triplets Leila, Liina, and Lily Luik, 30, who just started running seriously at age 24, trained together. They are the first ever triplets to compete in the same event at one Olympics.
3. Aria and Makenzie Fischer
Athletic ability certainly runs in the Fischer family. Two of the youngest players on Team USA, Aria and Makenzie Fischer swim for the U.S. water polo team; 17-year-old Aria and 19-year-old Makenzie picked up their interest in water polo from dad, Erich Fischer, who was on the 1992 U.S. team in Barcelona.
4. Kelly and Courtney Hurley
Kelly and Courtney Hurley's parents met while fencing and they passed their love of the sport on to their daughters. This is the second time the sisters are competing for the United States; in London, they took home bronze in the Women's Team Epee.
5. Gary and Paul O'Donovan
After clinching the silver medal in the men's lightweight double sculls and becoming the first Irish athletes to medal in rowing, brothers Gary and Paul O'Donovan have become a social media sensation thanks to their witty interviews. Their biggest complaint about winning? Being stuck in Rio and missing the celebrations in the pubs in Ireland.
5. Jamie and Andy Murray
British tennis players Jamie and Andy Murray lost their first match in the doubles tournament, but Andy won a gold, becoming the first singles player to win back-to-back gold medals. He captured our hearts when reminding a reporter that he is not the first tennis player to win two gold medals; that distinction is held by the Williams sisters.
6. Chloe and Max Exposito
The Australian brother and sister — the first two to make the team — became interested in modern pentathlon thanks to their dad's encouragement; he competed in the event for Australia in the 1984 Los Angeles Olympics.
7. Julia and Katie Reinprecht
The sisters, making an appearance in their second Olympic games with the U.S. field hockey team, learned the sport from their mom, who also coaches field hockey.
8. Jared and Rachel Tallent
While Jared medaled in race-walking at the London Olympics, Rio marks sister Julia's first time race walking in the Olympics for Australia. She should do well if she listens to her older brother — not simply for brotherly advice, but also because he's her coach.
9. Callum and Derek Hawkins
While Callum and Derek Hawkins train together with their dad as coach, they also view each other as rivals. They both qualified for the British Olympic marathon team after strong finishes at the London marathon.
10. Bronte and Cate Campbell
In the 2012 London games, Bronte and Cate Campbell became the first sisters to represent Australia in the same event during Olympic competition. And the two make a formidable team: In Rio, the sisters won gold in the women's 4x100 freestyle relay.
12. Barbara and Pol Pla
While sister Barbara, 33, boasts a decade more experience playing rugby than brother Pol, 23, they both qualified for their respective Spanish rugby teams.
13. Richard and Peter Chambers
The British brothers grabbed silver in the London Olympics for the lightweight four crew for rowing and are back again to row in Rio.
14. Josh and Nathan Katz
At 18, Josh Katz is the youngest Australian to qualify for judo during the Rio games. He followed in Nathan's footsteps in the sport and the two are competing in Rio together for the first time.
15. Bill and Niall Williams
While Bill is known in New Zealand for playing with the All Blacks, little sister Niall also boasts serious rugby skill, which she developed from keeping up with Bill on the pitch. Both made their nation's rugby sevens team, but Bill ruptured his Achilles tendon, ending his run early.
16. Jacob and Graeme Saunders
These Canadian brothers are participating in their first Olympics during the Rio games, sailing in the two-person 470 race on a boat they call Chester, named after their hometown, which supported their sailing career.
17. Arina and Anastasia Rodionova
While Australian sisters Arina and Anastasia made their Olympic debut in doubles tennis in Rio, they were eliminated in the first round.
18. Jonny and Alistair Brownlee
Alistair Brownlee won gold in the triathlon in the 2012 London games. His success causes brother, Jonny, also a triathlete, to push himself harder.
19. Lucas and Moises Duque
The Brazilian rugby playing brothers felt proud to take the field at home this Olympics.
20. Felipe and Daniel Sancery
The Duque brothers aren't the only ones playing on the Brazilian rugby team. Felipe and Daniel Sancery also took the rugby pitch for their national team, marking the first time the identical twins participated in Olympic competition.
21. Emma and David McKeon
While dad, Ron, swam for Australia in the 1980 and 1984 Olympics, Emma and David McKeon are making their own mark in Rio. During her first Olympic performance, Emma won a gold, two silvers and a bronze medal.
22. Michael and John Whitaker
At age 61, John Whitaker is one of the oldest Olympians in Rio. He first won silver for Great Britain in the 1984 games for show jumping. Brother, Michael, 56, is competing in his fifth Olympics. The brothers prove that age is just a number.
23. Katherine and Michelle Plouffe
During the London Olympics, Katherine struggled while watching sister Michelle play basketball for Canada. Both twins qualified for the Canadian team for Rio, meaning Katherine won't be squirming in her seats this time.
24. Margaux and Isabella Isaksen
While older sister Margaux's attendance in Rio marks her third Olympic performance, Isabella joined for her first appearance in the games. Both women compete in the modern pentathlon, in which Margaux is ranked No. 1 in the world.
25. Rieko and Akira Ioane
Both of Rieko and Akira's parents played rugby and the brothers joined New Zealand's rugby team in Rio for their first Olympic appearance.
26. Sharon and Nicole Acevedo
Rugby playing certainly seems to run in families with Sharon and Nicole playing in the Rio games, representing Colombia.
27. Becky and Ellie Downie
28. Erik and Kawika Shoji
The Hawaiian brothers are making their Olympic debut on the U.S. men's indoor volleyball team. The brothers both played volleyball at Stanford prior to beginning their Olympic career.
29. Lisa and Anna Hahner
These German twins came under fire for holding hands while crossing the finish line of the marathon in Rio. While they did not do as well as they hoped, they were grateful to finish hand in hand.
30. Caroline and Carmen Marton
For 26-hours a week, Caroline and Carmen Marton fight. It's not because of a sibling squabble; it's just how the two Australian sisters prepared for competing in taekwondo in Rio. While Carmen participated in Beijing and earned a medal in London, Rio marks Caroline's first games.
31. Juliana and Fernando Gaviria
These Colombian siblings both compete in cycling track and Fernando was named a Latin American athlete to watch during the Rio games.
32. Paul and Mark Gleghorne
While the brothers grew up in Ulster in Northern Ireland, they're playing for different field hockey teams during the Olympics — Paul for Ireland, Mark for Great Britain.
33. Omar and Amro El Geziry
Modern pentathlon is really a family affair for the El Gezirys. While the Egyptian brothers are the first brothers to compete in the event in the same games, Amro is married to Isabella Isaksen, who is competing in the individual pentathlon for Team U.S.A.
34. Florencia and Agustina Habif
These Argentinian sisters play field hockey; Florencia already has an Olympic silver, while Agustina is hoping to earn her first medal.
35. Sanne and Lieke Wevers
These twins represent the Netherlands in gymnastics and Sanne gained attention after besting Simone Biles to win gold on the balance beam. This makes her the first Dutch gymnast to win any medal during the Olympics.
36. Deniz and Ates Cinar
These brothers sail for Turkey and Rio marks their third Olympics together.