Oh, for the love of pete, that’s so what I started out looking for in my head, d’oh!
C’mon Twicks you gotta come back and tell us which ones you’ll be using. Any of the real siblings playing sibs would be all tricky-like, pretty please?? 
Sorry to be picky, but Ruby Dee was not Sidney Poitier’s sister in “Raisin in the Sun”- she was his wife.
Oops! Thanks for the correction.
As far as which ones I’m going to use – no final decisions yet, but the choices have to be relatively mainstream, so some of these are just too hip for the room. Right now I’m liking Hayley Mills-Hayley Mills, Pacino-Caan, Brando-Steiger, Kinnear-Ford (though I may do Bogart-Holden for that film), Cage-Aiello, Depp-DiCaprio, Russell-Paxton-Elliot, Hammill-Fisher. I need an even dozen in all, and at this point I should go back and look at the list I started with.
Keep 'em coming, gang!
Judge Reinhold and Jennifer Jason Leigh
Fast Times at Ridgemont High
Edward Norton and Edward Furlong
American History X
Ralph Bellamy and Don Ameche
Trading Places
Al Pacino and Mary Elizabeth Mastrantonio
Scarface
Kristin Scott Thomas wasn’t Hugh Grant’s sister in that movie?
She’s the one who he woke up hurriedly (or she woke him up? I so can’t remember the specifics) to attend the first wedding, at the very beginning, for which they were late, right? The one who ended up marrying that cowboy?
I thought they seemed to be bro/sis to me.
On a related subject, Hugh Grant and Emma Thompson
Love Actually
Oh, I see someone else said that.
Ryan Phillipe and Sarah Michelle Gellar
Cruel Intentions
Real-life siblings playing real-life siblings:
James & Jeanne Cagney
as George M & Josie Cohan in Yankee Doodle Dandy
David, Keith & Robert Carridine
Stacey & James Keach
Dennis & Randy Quaid
as the Younger, James and Miller brothers in The Long Riders
Since Sean Bean / David Wenham has been mentioned, I have to mention Miranda Otto and Karl Urban (Eowyn and Eomer).
Sarah Michelle Geller & Michelle Trachtenberg(sp?) were magically sisters on Buffy.
Jennifer Garner and Mia Maestro play half-sisters on Alias.
I’m not going to name the whole Brady Bunch, Eight is Enough or Party of Five folks.
Brian
Anthony Michael Hall and Dara Barron in National Lampoon’s Vacation.
Dana Hill and Jason Lively in National Lampoon’s European Vacation.
Juliette Lewis and Johnny Galecki in National Lampoon’s Christmas Vacation.
Ethan Embry and Marisol Nichols in National Lampoon’s Vegas Vacation.
Since I saw that group but not this one,
Kevin Costner, Linden Ashby, Michael Madsen
Wyatt Earp
Kevin Costner and Scott Glenn
Silverado
David Carradine and Michael Madsen
Kill Bill, primarily vol II
Geena Davis and Lori Petty
A League of Their Own
Kirsten Dunst and Bradley Pierce
Jumanji
Rupert Grint, James Phelps, Oliver Phelps, Chris Rankin, Bonnie Wright
the various Harry Potter movies
Scott Caan and Casey Affleck
Ocean’s Eleven and Ocean’s Twelve
Colin Farrell and Gabriel Macht; also Scott Caan, Gregory Smith and Will McCormack
American Outlaws
Jeremy Piven and Daniel Stern
Very Bad Things
Renee Zellweger and Bonnie Hunt
Jerry Maguire
Matt Damon and Greg Kinnear
Stuck on You
Laura Linney and Mark Ruffalo
You Can Count on Me
Richard O’Brien and Patricia Quinn, Rocky Horror Picture Show
Cliff De Young and Cliff De Young, Shock Treatment
Diana Scarwid and Xander Berkeley, Mommie Dearest (adults)
Mara Hobel and Jeremy Scott Reinbolt, Mommie Dearest (children)
Parker Posey, Josh Hamilton and Freddie Prinze Jr, The House of Yes
Just wanting to point out that I absulutely LOVE this movie. And the screenplay.
I’ve checked three times and it looks like no one (including me!) remembered:
Matthew Broderick and Jennifer Grey in
Ferris Bueller’s Day Off
:smack
Couple more:
Jason Patric & Corey Haim; Corey Feldman & what’s-his-name Jamison Newlander in
The Lost Boys. I had to look up the 2nd Frog brother, though.
And an old one:
C. Thomas Howell, Patrick Swayze and Rob Lowe
The Curtis brothers in The Outsiders
Joan Cusack and Robin Williams Toys…in which they turned out to be…Not exactly siblings, but still “siblings”, by “adoption,” sort of. She was a life-sized toy. An android.
How about Robert Duvall & Robert De Niro in “True Confessions”?
Good one, astorian – one of my sister’s favorite movies.
Speaking of people playing their own twin: Jeremy Irons and Jeremy Irons
Dead Ringers.
An excellent (albeit very creepy) movie, which, for some reason, I have to look up the title of every. single. time
Joan Jett and Michael J. Fox
Light of Day