Team USA's Venus and Serena Williams were knocked out of the tennis doubles competition in a shocking upset, dashing the duo's hopes for a fourth Olympic gold. Serena is still vying for the gold medal in women's singles, however.
Source: NBC Olympics
Fencers Kelley Hurley (left) and younger sister Courtney were the first set of siblings to qualify for US Olympic team in 2016. Rio is their second time competing at the Olympics together.
Source: USA Today
Estonian marathon runners Leila, Liina, and Lily Luik will make history as the first Olympic triplets to compete together.
Fun fact: The sisters, now 30, only started running seriously at age 24.
At just 17 and 19, Aria Fischer (left) and sister Makenzie are among the youngest athletes on the US water polo team. Their father was a water polo Olympian, too.
Modern pentathletes Chloe Esposito and younger brother Max were the very first athletes to qualify for the Australian Olympic team.
Source: ABC
Jamie Murray (left) and younger brother Andy play tennis doubles for Great Britain. Unfortunately, the pair lost their first match in Rio and won't be advancing as a team — but Andy is still in the men's singles competition.
Source: BBC
Michael Whitaker (left) and younger brother John, of Great Britain, are legends in the equestrian world. Rio is their fourth Olympics together.
Source: The Horse Rider's Journal
Americans Isabella Isaksen (left) and older sister Margaux compete in modern pentathlon, a sport that combines fencing, pistol shooting, horseback riding, freestyle swimming, and cross-country running.
In the London Games, British triathletes Jonny Brownlee (left) and older brother Alistair took home the gold and bronze medals, respectively. Now, they're preparing to dominate the podium in Rio.
Source: Team GB
Becky Downie (left) and older sister Ellie are gymnasts on Team Great Britain. The two share a kitten named Rio in honor of the 2016 Summer Games.
Source: NBC Olympics
Julia (left) and Katie Reinprecht made their Olympic debut with the US field hockey team in London, and they're back for more in Rio. Unsurprisingly, their mother is a field hockey coach.
Hawaii natives Erik Shoji (left) and older brother Kawika are making their Olympic debut on the men's indoor volleyball team.
Source: Honolulu Star-Advertiser
Jared Tallent and younger sister Rachel Tallent compete in racewalking for Australia. (Yes, there really is an Olympic event dedicated to fast walking!)
Callum Hawkins (left) will run the marathon for Great Britain alongside older brother Derek.
Source: BBC
Australian swimmers Bronte Campbell (left) and older sister Cate kicked off the competition in Rio with a gold medal in the women's 4x100 freestyle relay.
Source: NBC Olympics
33-year-old Spaniard Barbara Pla has more than a decade of pro rugby experience. Her younger brother Pol is making his Olympic rugby debut in Rio.
Great Britain's Richard Chambers (left) and younger brother Peter rowed their way to a silver medal at the London games.
Source: Belfast Telegraph
Nathan Katz (left) and younger brother Josh are judo fighters for Australia. The brothers originally hoped to make the 2020 Olympic team, but performed so well this year that they were selected for Rio.
Source: Huffington Post
Sonny Bill Williams and younger sister Niall also play rugby for New Zealand. (Sadly, Sonny Bill suffered an Achilles tendon injury Tuesday that ended his participation in the Rio games.)
Source: The Guardian
Australians Arina Rodionova (left) and older sister Anatasia made their Olympic debut as a tennis doubles pair in Rio, but were knocked out of competition in round one.
Source: Rio 2016
Lucas Duque (left) and younger brother Moises are one of two sets of brothers playing rugby for Brazil.
Source: Rio 2016
Felipe (left) and Daniel Sancery are the second set of siblings on Brazil's rugby team — only these two are twins.
David McKeon and younger sister Emma swim for team Australia. Emma's already won a gold and bronze in Rio.
Source: Rio 2016
Australian taekwondo fighters Caroline Marton (left) and younger sister Carmen train for four hours together every day.
Jacob Saunders (left) and older brother Graeme are sailors from Canada. Rio marks the brothers' Olympic debut.
Source: CBC News
Egyptian athletes Omar El Geziry (left) and younger brother Amro, are the first brothers to compete in modern pentathlon in the same Olympic games.
Twin sisters Katherine (left) and Michelle Plouffe are on Canada's basketball team. This is their first time competing at the Olympic level.
Source: Edmonton Sun