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Blake Lively and Penn Badgley, 'Gossip Girl' - The two were romantically involved when they played step-siblings in the show, which might seem less weird considering that their characters also got together.

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Blake Lively and Penn Badgley, 'Gossip Girl' - But that only meant that after the two had broken up in real life, they had to keep playing lovers for the duration of the show. 

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Matthew Broderick and Jennifer Grey, 'Ferris Bueller’s Day Off' - Jennifer Grey played Ferris’ sister, and their antagonistic on-screen relationship was apparently fueled by their off-screen chemistry. Unfortunately, they ended badly.

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Matthew Broderick and Jennifer Grey, 'Ferris Bueller’s Day Off' - In 1987, the pair was in a car crash that killed two people, caused by Broderick who was driving on the wrong side of the road. The traumatic experience altered Grey's career and ultimately ended their relationship.

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Will Arnett and Amy Poehler, 'Blades of Glory' -

The married couple took on the role of a  creepily incestuous sibling duo in the 2007 ice skating comedy.

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Will Arnett and Amy Poehler, 'Blades of Glory' - They did such a good job at making viewers laugh and cringe, but unfortunately the pair divorced a few years later. 

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Maureen McCormick and Barry Williams, 'The Brady Bunch' - The wholesome show had a strange secret between Marcia and Greg, who were getting closer than their characters would be comfortable with off-screen.

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Maureen McCormick and Barry Williams, 'The Brady Bunch' - McCormick recounted her exact thoughts when first making out with Williams in her 2008 memoir: "Oh my God! I’m kissing my brother. What am I doing?"

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Drew Barrymore and David Arquette, 'Never Been Kissed' - The movie siblings had a very quiet romance, but Arquette revealed on 'Watch What Happens Live!' that she was one of the kindest people in Hollywood and that, “[they] used to date when we were very young kids. Well, not little kids!”

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Emily VanCamp and Chris Pratt, 'Everwood' - Before Pratt was an A-lister, he dated his on-screen sister for three years.

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Dean Charles-Chapman and Nell Tiger-Free, 'Game of Thrones' - Much like the Lannister family, the actors behind siblings Myrcella and Tommen Baratheon became romantically involved, doubtlessly making them the butt of many jokes.

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Robert Pattinson and Nikki Reed, 'Twilight' - Before Kristen Stewart, there was Nikki Reed, also known as Edward Cullen's adoptive sister. Imagine navigating that love triangle later on set—yikes.

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Ashley Greene and Kellan Lutz, 'Twilight' - Things got steamy on the 'Twilight' set, especially between the Cullen siblings played by Greene and Lutz. They remained friends after breaking up.

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David Cassidy and Susan Dey, 'The Partridge Family' - Dey had a huge crush on Cassidy, who reportedly paraded his other dates in front of her until they finally ended up having a one-night stand that went nowhere. 

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Michael C. Hall and Jennifer Carpenter, 'Dexter' - The actors play adopted siblings, but they ended up in a real-life romance and even eloped in 2008. 

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Michael C. Hall and Jennifer Carpenter, 'Dexter' - They separated a few years later, around the same time the writers had arranged for their characters to have an incestuous story line, which the divorcees then had to play out.

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Selena Gomez and David Henrie, 'Wizards of Waverly Place' -

Back in 2010, Gomez was linked with her on-screen brother, Henrie. It didn't last, but the two remain close friends.

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Ian Somerhalder and Maggie Grace, 'Lost' - The sexual tension between stepsiblings Boone and Shannon was an early plot point for 'Lost,' and was apparently fueled by a real, but short-lived, chemistry.

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Lauren Graham and Peter Krause, 'Parenthood' - The actors first met costarring in 'Caroline in the City,' but fell in love while playing brother and sister on the 2010 show 'Parenthood.'

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Lauren Graham and Peter Krause, 'Parenthood' - They had to keep playing siblings until the show's end in 2015, but they have stood the test of time. Graham even got Krause a role on the 2017 'Gilmore Girls' revival as the park ranger!

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Emily VanCamp and Dave Annable, 'Brothers & Sisters' - VanCamp seems to like her on-screen siblings, and dated her TV brother Dave Annable. The writers were kind enough to add a plot where it turned out the two weren’t related after all.

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Katherine Heigl and Jason Behr, 'Roswell' - The two dated briefly in 1999 when they played siblings on the TV show 'Roswell.'

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Evan Rachel Wood and Shane West, 'Once and Again' - Wood first played West’s on-screen sister when she was just 12. Several years later, when she was an adult, the two were spotted getting cozy at West’s 31st birthday party.

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Kirstie Alley and Parker Stevenson, 'North and South: Book II' - The pair had been married for three years before they played brother and sister Billy and Virgilia Hazard in the 1986 miniseries.

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Mark Hamill and Carrie Fisher, 'Star Wars' - Luke and Leia famously shared a kiss before they knew they were siblings, but it turns out Hamill and Fisher shared a kiss while well aware that they'd be playing siblings!

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Mark Hamill and Carrie Fisher, 'Star Wars' - Hamill revealed how it had transpired, involving a bit of liquid courage. He told her, "I think I’m a fairly good kisser," and then, "next thing you know we’re making out like teenagers!”

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Mark Hamill and Carrie Fisher, 'Star Wars' - Thankfully, just as Luke and Leia never continued their romance after discovering they were related, Hamill and Fisher eventually developed a sibling-like relationship themselves.

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Jamie-Lynn Sigler and Robert Iler, 'The Sopranos' - Meadow and A.J. Soprano were seen being quite affectionate together off-screen, though they later denied they were involved.

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Jason Priestley and Shannen Doherty, 'Beverly Hills, 90210' - Brenda and Brandon Walsh were spotted smooching at the 1991 Emmy Awards, shocking fans of the show.

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Charlie Day and Mary Elizabeth Ellis, 'Reno 911!' - The 'It's Always Sunny' actors met in 2001 and played inbred siblings on 'Reno 911!' in 2004, after they had already started dating.

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Charlie Day and Mary Elizabeth Ellis, 'Reno 911!' - Day told Monster Fresh, "Nothing says, 'This is the girl I want to marry' like pretending she's your sister and making out with her to get a job."

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