Incestuous Casting

The rarer and far grosser flipside to Cast Incest, where people who are Real Life Relatives have to play romantic partners. This scenario is more common in community and High School theater productions where there is a limited pool of actors to choose from; while producers may be manipulative and hate filled, they aren't invulnerable to Squick. It also happens a bit in figure skating.

Examples of Incestuous Casting include:

Anime

In the Latin America (re)dub of Revival of Evangelion, Shinji is voiced by Victor Ugarte, and due to Executive Meddling, the voice of Rei is replaced by Gaby Ugarte. Which becomes rather awkward during that scene where Shinji enters Rei's room just after she's finished bathing.
Robotech‍'‍s Khyron and Azonia, played by Greg Snegoff and his mother Alexandra Kenworthy.

Comic Books

Y: The Last Man has an in universe example, where the protagonist mentions him and his sister being cast as Romeo and Juliet, respectively, in the school play. However, they both decide they want to avert it, and his sister is selfless enough to let him drop out of the play.
During Marvel's Civil War storyline, Johnny Storm and Susan Storm Richards, brother and sister members of the Fantastic Four, assumed undercover identities as husband and wife. Sue considers it the most disturbing thing she's ever done.

Film

Show Within a Show example: The movie musical Royal Wedding is about a brother and sister team who play lovers on stage. Fred Astaire and Jane Powell were of course not related.
In Freaks (1932), the dwarf main characters, who become a couple, were played by siblings Harry and Daisy Earles.
- Also true of the dwarf couple in Putney Swope.
Siblings Christopher Khayman Lee and Chyler Leigh kissed in the 1999 tv series Safe Harbor and in the 1997 movie Kickboxing Academy.
There has been at least one case where two porn actors (Amber Lynn and Buck Adams) refused to do a scene together because, unknown to anyone, they were siblings.
In-universe example: In High School Musical, one character remarks that if Sharpay could find a way to play both Romeo and Juliet, Ryan (her brother) would be out of a job, suggesting that the two have played romantic partners in the past.
- It's also said that the two of them always got the lead roles in all the productions before Troy and Gabrielle, so it's safe to assume at least one of these would be a romantic pairing.
The German semi-fictional movie about the titular Comedian Harmonists has siblings Meret and Ben Becker playing love interests, including an long and dramatic Kiss Scene.
In Primer, the married couples are played by brother and sister.
Real-life porn stars Britney and Whitney Stevens typically tease this trope in the near-dozen scenes they've done together by casually going as far as "Second Base" and nothing further...on-camera, but once admitted they got drunk enough off-camera to go all the way with each other. Didn't bother them that much, just reminding themselves to drink a bit more responsibly.
In the Finnish dub of Toy Story 3, Barbie and Ken are played by siblings Krisse and Eppu Salminen.
Averted at the last minute with Bonnie and Clyde: Warren Beatty initially cast Shirley MacLaine as Bonnie, but replaced her when he opted to play Clyde himself. The reason? He didn't want to have to kiss his sister. (Yes, they're siblings—in this life.)
Manos the Hands of Fate's infamous ending gets even worse when you remember that The Master and Debbie are played by real life father and daughter.
Hocus Pocus: "The Master"—no, not the same Master as the one in the example above—and his bickering wife are played by brother and sister Garry and Penny Marshall in uncredited roles.

Literature

In-universe in The Family Fang, the siblings who play romantic leads together are rightfully horrified when they find out their parents set this up. It's . . . complicated.

Live Action TV

Dancing With The Stars- Real-life siblings Julianne and Derek Hough have danced romantic numbers together several times
Arrested Development - Justine Bateman, sister of Jason Bateman, plays a prostitute who propositions Jason's character. It's also played with, where Jason's character believes that Justine's character is his sister - which turns out to be false. Appropriately enough Michael comes out with this line.

Marry me! ... that's wrong on so many levels.

One Life to Live - Real-life siblings Kristen and Eddie Alderson play unrelated characters. A few years back, a storyline involved Eddie's Matthew having a crush on Kristen's Starr. The boy had to pretend to have a crush on his sister. Gross.
Averted on Rumpole of the Bailey: Mrs. Rumpole occasionally accuses Rumpole (played by Leo McKern) of having an affair with Liz Probert, who for Series 5-7 is played by Abigail McKern (Leo's daughter). However, he never actually does anything of the kind.
An in-universe example: Justin and Alex from Wizards of Waverly Place, who are brother and sister, pretend to be married and have three kids, in one episode.
In another in-universe example, the Saturday Night Live sketch "Red Ships of Spain" had the in-show version of Robert Goulet (played by Will Farrel) starring in a broadway show where Goulet's romantic interest is played by his own daughter. The daughter seems very disturbed about the situation, but Goulet is totally oblivious to the implications.
Another in-universe example is It's Always Sunny in Philadelphia. The gang is putting on a musical Charlie wrote and he casts Mac in the lead role. Dennis and Mac end up switching parts forcing Dennis and his sister Dee to play romantic partners. They end up having to cut a kiss out because of this.
- One episode of It's Always Sunny in Philadelphia shows Frank (Danny Devito) in a restaurant opening leering at a waitress played by Danny Devito and Rhea Perlman's real-life daughter.
In Parralax, the Martin Dunkly from Ben's World (Kasmir Sas) is frequently hitting on the Melinda (Francoise Sas)from Ben's world (and occasionally the one from Katherine's world, when he mistakes her for the Melinda from Ben's world) and trying to get together with her. The two are brother and sister in real life.
Frank Sinatra, Jr., appeared in an episode of Adam-12" as a police officer seeking the affections of an unrelated woman played by his sister Tina Sinatra.
Weird sort-of example from Degrassi: Craig's little sister is played by Alex Steele. Alex is the real-life sister of Cassie Steele, who plays Manny. Thus, some viewers were squicked when Craig and Manny started hooking up.
- Lampshaded in a season two episode when Craig and Manny have their first date and Craig later says Manny reminds him of his little sister.

Music Videos

In the video for Britney Spears' debut hit "Baby One More Time", her love interest is played by her real life cousin, Chad Spears. Admittedly the "romance" never amounts to anything but suggestive glances across the basketball court and her singing to him briefly, but still, given the number of guys who appear in the video, did they really have to pick the one she was related to to play the part?
In the Michael Jackson / Paul McCartney video "Say, Say, Say", Michael's love interest is played by his sister LaToya.
In the music video for Stuck by Stacie Orrico, her cheating boyfriend is played by her real-life cousin, actor Trevor Wright.

Theatre

In the Gothenburg concert version of Chess, Freddie's and Florence's actors are brother and sister. Granted, most of what Freddie and Florence do onstage is argue, but still...
Historically, commedia dell'arte (Italian comedy) troupes were often families, so the Lovers could be played by brother and sister.
Fred Astaire and his sister Adele played romantic roles opposite each other for many years in stage musicals. When his sister, who was more popular, quit to get married he worried his career would be over. Of course, history had another medium for him to become famous in.

Toys

Barbie and Ken are often regarded as the perfect couple. So long as one doesn't dwell on the fact that the businesswoman who designed the first dolls (Mattel president Ruth Handler, way back in 1959) used her own daughter and son as models.

Web Original

In the Hillywood Show‍'‍s Dark Knight video, The Joker and his girl Harley Quinn are played by real-life sisters Hilly and Hanna Hindi. Yes, that's right. The Joker is a Sweet Polly Oliver and she nearly makes out with her sister.

Western Animation

On Family Guy, creator Seth MacFarlane plays Stewie, while his sister Rachael MacFarlane played his love interest Olivia in a few episodes. Both characters are babies, though, so maybe this is more a meta case of Father, I Want to Marry My Brother?
- And on American Dad, Seth plays Bill, Stan Smith's body double, and Rachael plays Stan's daughter Hailey. Guess who hook up in one episode ... And then it gets grosser when Stan finds out and has to replace Bill on a camping trip with Hailey. But don't worry- nothing happens.
In-universe example: In Avatar: The Last Airbender, siblings Katara and Sokka are able to visit a school in the Fire Nation by posing as a married couple, Wang and Sapphire Fire. And to add to the subtext, Katara's disguise makes her look heavily pregnant. (One wonders where she found the stuffing. More clothesline raids?)
In-universe in Doug—Doug got a role in a school play opposite his crush, Patty. Instead Patty had to drop out for some reason, forcing Doug's sister Judy to take the role.
The Italian dub of Total Drama Island has Heather and Trent voiced by Monica Ward and her son, Alessandro. They kissed in the episode "Search And Do Not Destroy".