(For a puzzle, as usual) – I need pairs of actors who played siblings, like:
Tom Cruise and Dustin Hoffman
Rain Man
Dan Aykroyd and John Belushi
Blues Brothers
Ruby Dee and Sidney Poitier
A Raisin in the Sun
Vivien Leigh and Kim Hunter
A Streetcar Named Desire
Whatcha got?
Christian Slater and Sean Connery.
(heh)
Greg Kinnear and Matt Damon
Stuck on You
Mark Hamill and Carrie Fisher
Does this one really need a spoiler? Star Wars IV-VI, obviously
Hugh Grant and Kristin Scott Thomas
Four Weddings and a Funeral
Seth Green and Vince Vieluf
Rat Race
Well, off the top of my head:
Arnold Schwarzenegger and Danny Devito Twins, and I am slightly ashamed that this was the first one I could think of
Mark Hamill and Carrie Fisher Revealed in Return of the Jedi
Bryan, did you mean Christian Slater and Kevin Costner, since the only thing I could find that Slater and Connery were in together was Robin Hood, Prince of Thieves where Slater played Costner’s illegitimate half brother Although I suppose it’s entirely possible I’m being wooshed and they played related characters in different movies. 
On that note, a case might be made for John Travolta and Michael Madsen Vega brothers in Pulp Fiction and Resevoir Dogs, respectively
Rosie O’Donnell and Andie MacDowell
Riding the Bus with My Sister, if you aren’t particular on details like anyone having seen the movie.
Kyra Sedgwick and Julia Roberts
Something To Talk About
What about real relatives? Martin Sheen and Charlie Sheen Wall Street Christian Slater and Helen Slater The Legend of Billie Jean Eric Roberts and Eliza Roberts Love is a Gun.
Sylvester Stallone and Terri Hatcher in Tango & Cash.
Weren’t Robin Williams and Harvey Firestein brothers in Mrs. Doubtfire?
Kurt Russell and Billy Baldwin in Backdraft.
my fav. is michael york and marty feldman.
Julia Roberts and Catherine Zeta-Jones
America’s Sweethearts
Sean Patrick Flannery and Norman Reedus Boondock Saints
Sean Bean and David Wenham Lord of the Rings
Beau Bridges and Jeff Bridges.
The Fabulous Baker Boys.
For that matter, Charlie Sheen and Emilio Estevez in Men at Work.
Al Pacino, James Caan, John Cazale (Pick two)
The Godfather. You could throw Robert Duvall in the mix, though he was adopted.
Marlon Brando, Rod Steiger
On the Waterfront. In which one brother shoulda looked out for the other just a little bit.
I think Ekers Christian Slater/Sean Connery pairing is from
In the Name of the Rose, where they were members of a religious brotherhood, not a familial one.
Nope, no way. Icky, icky, icky.
KST actually confessed her love to HG during the Scottish (3rd) wedding.
The late great Charlotte Coleman played HG’s sister. The always adequate James Fleet played KST’s brother.
Do half siblings count? If so,
James Earl Jones and Robert Duvall in A Family Thing
James Earl Jones and Gwyneth Paltrow in Jefferson in Paris
Tom Hanks and child-actor Jeffrey Scaperotta in You’ve Got Mail
Or same actor siblings:
Jackie Gleason, Jackie Gleason, and Jackie Gleason as the Justice Brothers in Smokey and the Bandit Two
Haley Mills and Haley Mills in The Parent Trap (and its sequels)
Full siblings:
Greg Kinnear and Harrison Ford in Sabrina
Paul Sorvino and (the late and ultimate “that’s what’s his name” tough-guy) Joe Viterelli in The Firm as the gangster Morolto brothers
One of the weirdest casting decisions for a pair of actors:
Robert Wagner and Spencer Tracy (his senior by 30 years) as brothers in The Mountain
Some personal favorites:
Anthony Hopkins, John Castle and Nigel Terry as the Plantaganet Boys in The Lion in Winter
Brendan Gleeson (the ultimate “that’s what’s his name” Irish tough-guy actor) and Brian Cox in Troy (the movie was “eh”, but their casting was perfect)
Charlton Heston and John “Anything for a Buck” Carradine in The Ten Commandments
Jack Lemmon and Kim Novak in Bell, Book and Candle
Vivien Leigh, Evelyn Keyes and Ann Rutherford in Gone With the Wind (also featuring Leslie Howard and Alicia Rhett (who had a 22 year age difference) as siblings as well as Rand Brooks & Olivia de Havilland and George “Superman” Reeve & Fred Crane as the identical Tarleton twins)
Odd TV siblings:
Lorne Greene and Robert Reed in Roots
Betty White, Roddy McDowell, Tommy Smothers and Carol Burnett in The Family segment of The Carol Burnett Show
I thought they were friends in that one.
John and Joan Cusack
Say Anything
Jake and Maggie Gyllenhaal
Donnie Darko
John C. Reilly and Maggie Gyllenhall
Criminal
Sean Bean and David Wenham
Lord of the Rings
Nic Cage and Danny Aiello
Moonstruck
Johnny Depp and Leonardo DiCaprio
What’s Eating Gilbert Grape?
A more obscure one; Charles Gray and Nicol Williamson
The Seven Percent Solution
[QUOTE=Murcielago]
Al Pacino, James Caan, John Cazale (Pick two)
The Godfather. You could throw Robert Duvall in the mix, though he was adopted.
Don’t forget their sister, Talia Shire.
Kurt Russell, Bill Paxton, Sam Elliott
Tombstone
Hugh Grant, Emma Thompson
Love Actually
Emma Thompson, Kate Winslet
Sense and Sensibility
Macaulay Culkin, Mandy Moore
Saved!
Kieran Culkin and Ryan Phillipe
Igby Goes Down
Rory Culking and Peter Anthony Tambakis
Igby Goes Down - younger versions of the same characters