In Get Backers, Juubei and Sakura Kakei are a borderline case: they're always on the same side (even when their "most important persons" aren't), but they're rarely seen actually working together. Himiko and Yamato are like this in flashbacks, but later revelations reveal that this is actually a more appropriate (and manga-only) description of Himiko and Ban.
Ran from Telepathy Shoujo Ran gets a lot of support from her older brother Rin, who is a very capable Amateur Sleuth. She bickers mostly with fellow telepath Midori though.
Tweedledee and Tweedledum in Kiddy Grade. Also fraternal twins.
More like cousins. And they never really formed a permanent team. However, Mayday and her baby brother Benjy have the makings of becoming one in the future.
In Excalibur Alistaire and Alysande Stuart, a military officer and a scientist working for the Weird Happenings Organisation.
Captain Britain and Psylocke, although they did not team up that often.
Helena and Damian Wayne, alternate universe half-siblings and Batman's two biological children, seem okay with this dynamic after their initial meeting. They called each other "brother" and "sister," which is more than they've given their three adopted brothers and adopted sister.
In Paperinik New Adventures, while Ziggy left the PBI because he was tired of secrets,he still has his skills and occasionally works with his sister Mary-Ann,who is still an agent.
The Spy Kids films. Possibly parodied in the sequel which introduces a rival team also composed of a brother and sister.
In Pacific Rim, this is one of the possible configurations to operate a Jaeger. The closest known example would be the pilots of Chrome Brutus, Zeke Amarok and Ilisapie Flint, who were male-female cousins.
Top Dollar and Myca in The Crow are a villainous variation of this. They're the closest members in Top Dollar's criminal syndicate, and she serves as his advisor on supernatural matters. Because they're villains, their relationship is openly incestuous.
Harry Potter's friends, the Weasley family, are an all-family team, but Ginny and her brothers work together regularly.
Princess Andromeda and Sir George (actually Georgina) in One Good Knight from Mercedes Lackey's Tales of the Five Hundred Kingdoms. Andie proposes that they become blood-siblings to invoke this trope, since otherwise The Tradition (the ambient magic in the land) would do everything it could to make her fall in love with the knight who rescued her.
Lackey's books in general have a tendency do do this whenever the main character isn't a total orphan, which is rare. Notably in the series with Andre Norton, where there are siblings everywhere.
In Dead Until Dark, he doesn't believe her when she tells him she was molested, because the molester was their uncle and seemed like a nice guy.
In Dead To The World, he volunteers Sookie to babysit an amnesiac vampire who's being headhunted by vicious witches, so he can get a cut of the reward money. His contribution was to loan Sookie a shotgun.
In From Dead To Worse, after his manipulation and cowardice forces her to smash a friend's fingers with a brick, she finally tells him she never wants to see him again.
Alec and Isabelle Lightwood in the Mortal Instruments. Also, there's Jace Wayland, who's technically their foster brother, but that tends to be over looked.
The 39 Clues has Dan and Amy Cahill working together to find Clues. Also applies to one of the rival teams in the Clue hunt: Ian and Natalie Kabra.
Carter and Sadie Kane take on the Egyptian gods in The Kane Chronicles. They start the series out extremely distant however, because they've been living apart for the better part of their formative years. The internal narration reveals that they're both still extremely loyal to each other despite that. And over the course of the series their team only gets stronger.
The Earth's Children series has Iza and Creb in the first book—aging sister and brother who are allowed to set up a hearth together. She's the clan medicine woman, and he's the clan shaman. Then in the third book, the Mamutoi custom is for the co-leaders to be a brother and sister. If somebody doesn't have a sibling and wants to set up a separate cave, they may adopt one.
Georgia and Shaun Mason from the Newsflesh trilogy. They were adopted as babies and both are born from different parents. They checked.
Jules and Yvette D'Alembert from E.E. "Doc" Smith's Family D'Alembert series. The fact that they achieved the two highest possible scores (1000 and 999) in their final training evaluation means they're evenly matched for ability, neither of them has to carry the other, and it doesn't exactly hurt their ability to get the job done.
Cashmere and Gloss, District One's tributes in the 75th Hunger Games.
Live Action TV
Game of Thrones: Margaery Tyrell and her brother Ser Loras form this in their three-way marriage to King Renly Baratheon in Season 2. Unlike Queen Cersei and King Joffrey's barely concealed discord in "Valar Dohaeris," the Tyrell siblings are very much in sync during the dinner conversation.
Luke and Sky in The Sarah Jane Adventures started to become this in the final episode. It's a pity we didn't get to see more.
In Stargate Atlantis, McKay's little sister Jeannie pops up a few times to help him solve various problems. They have great sibling chemistry (read: they're pretty mean to each other), largely helped by the fact that the two actors are siblings in real life.
Inverted in Merlin, in which Arthur and Morgana work together several times in the early seasons, but are estranged by the time they realize that they're half-siblings.
JAG: When in Russia, Harm & Mac gets help from such a team in "Gypsy Eyes".
In Dexter, the title character and his younger sister Debra team up in the seventh season after she learns of his secret and helps cover for him at times. Of course, this ends in the season 7 finale.
Sookie and Jason Stackhouse from True Blood work together and have a great relationship; although usually they have independant plotlines, when they do team up they're a pretty awesome team. For proof, just see the Fellowship of the Sun from season 2!
In classical music you have Wolfgang and Maria Anna Mozart.
Likewise Felix and Fanny Mendelssohn. Society being what it was, in both Mozart's and Mendelssohn's case the female half of the team was not allowed to pursue her career to the full extent.
The White Stripes consists of Jack White on guitar and vocals with his sister (well... ex-wife, but billed as his sister) Meg on drums.
The Carpenters: Karen and Richard.
Early on, No Doubt had Eric and Gwen Stefani. Eric left after the completion of "Tragic Kingdom" to become an animator on The Simpsons.
Orestes and Elektra, already mentioned under Theater.
Orestes and Electra of Electra are a brother-sister team, but not the kid-friendly sort. Their principal objective is to kill their mother and step-father for killing their father and treating them badly.
Iji takes point against the alien attackers, while her brother Dan acts as Mission Control. One of the subplots is about how Iji's love for her brother keeps her going despite Dan's assertions of emotional detachment.
World of Warcraft Cataclysm has two villainous brother-sister teams of dragons. Nefarian and Onyxia, who were fought separately before, appear together in Blackwing Descent, and the Bastion of Twilight has Valiona and Theralion.
Apollo Justice: Ace Attorney features titular character Apollo and his assistant/legal aide, Trucy, who are revealed to be half-siblings through their Missing Mom near the end of the game. The characters themselves remain unaware of this fact.
In Umineko no Naku Koro ni, Lambdadelta makes Bern accept the rule that if Ange enters the board as Bern's piece, Battler cannot find out her identity or else Ange dies specifically in order to avoid this trope. Ange is forced into a Mysterious Protector role instead.
The Dakota twins in Red vs. Blue are this, conforming to the traditional understanding of the trope most in the Freelancer saga. Brutally, horribly present especially in the current (post-Blood Gulch) era. Of course, this was The Director'splan.
As of season 10, Carolina and Epsilon, to a degree. Both are "children" of the Director, now teamed up.
Kim Possible and the tweebs in season four just barely fits the team trope. They outfitted the Kim-mobile with all its neat gizmos, and were very displeased when Motor Ed blew their doors off.
Ron and Hanna Stoppable fit the trope in Season Four as well.
Shego's Back Story also fits the trope as she and her brothers formed the heroic Team Go.
In Legend Of The Dragon the twins are a team at the beginning until Ang gets the power of the Golden Dragon. Ling, jealous, takes off, and in her rage and confusion, meets up with the Zodiac Master, and turns into the Shadow Dragon. But at the season finale, Ling comes back, and they end up sharing the power of Golden Dragon, thence to take out the Zodiac Master and his boss together.
Season 2 had yet another example in Yin Wi and the Emperor of the Darkest Yin.
from Super Friends, Zan and Jayna, despite being the Wonder Twins, also fit this trope.
He-Man and She-Ra, from their respective cartoons, are fraternal twins. Though they live on separate planets and usually operate independently, they fit this trope on the occasions when they work together, such as in the She-Ra pilot and the Christmas special.
Edward the Elder and Ęthelflęd Lady of the Mercians the son and daughter of Alfred the Great. Together as King of Wessex and Queen in all but name of the English Midlands they continued their father's good work of driving back Viking raiders and bringing the Danelaw back under English control.
Rukhsana (sister) and Adaz (brother) Kauser helped fight off Islamist extortioners at their farmhouse on the Indian-Pakistani border. While they were busy beating up the parents Rukhsana seized an ax and whacked one on the head with it. Then the two siblings held off the terrorists with captured rifles. According to The Other Wiki, one of them was a "distinguished" terrorist with a 4000 pound bounty on his head.
Tennis players Marat Safin and Dinara Safina are the only brother and sister ever to have reached the world #1 ranking in singles. They also made up the Russian team at the 2009 Hopman Cupnote An annual team event in which nations field teams of one man and one woman, with matches consisting of men's singles, women's singles, and mixed doubles. and finished as runners-up.