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Lisa Rinna and Patrick Muldoon, 'Days of Our Lives' -

It certainly wasn't easy for Lisa Rinna and Patrick Muldoon when they were working together on 'Days of Our Lives' back in the '90s. Rinna played Billie Reed on the soap opera, while Muldoon played her on-screen brother, Austin Reed. Decades later, while being interviewed about her time on the show, Rinna revealed that she had an affair with Muldoon! Although she later corrected herself: "Well, not an affair, but a couple of one-night stands." If you're into watching re-runs of '90s soap operas, you'll never see this one the same way again. 

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Blake Lively and Penn Badgley, 'Gossip Girl' -

The two were romantically involved when they played stepsiblings on 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. “We’ve pretty much gotten used to the response of, ‘Ew, that’s weird. That’s creepy,’” he told TV Guide.

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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. Check out these other celebs who got together on set.

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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 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 2016 '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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