"It's not like Janelle was raised under my dad's roof, and it wasn't like I was raised under her mom's roof. We're not siblings. Gah, I just—I never even thought about how weird that was, cause you and I knew each other long before our parents knew each other."
—Kody about his relationship with wife Janelle, Sister Wives
Usually, when writers create sibling romantic relationships, if they go the route of Not Blood Siblings or Flirty Stepsiblings, they have them be stepsiblings first. This adds to the Forbidden Fruit appeal and increases the tension.
Some stories, on the other hand, do it differently. If two characters are already in a relationship (sometimes even having kids of their own), and their parents get together, things will understandably be awkward. Note that a relationship between the parents doesn't need to actually happen, just the characters discussing/imagining the possibility is enough.
Related to Double In-Law Marriage and Tangled Family Tree, though both couples don't need to be married.
Examples:
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Fan Works
- All Assorted Animorphs AUs: Defied in "What if Peter and Naomi got married?". Jordan already has a crush on Marco, so when Marco learns they're going to become stepsiblings, he acts as unappealing as possible to turn her off him.
- A Thing of Vikings: Picknose Rolfsson is already the lover of both Signy and Mairwen before their mother rekindles her relationship with his father. This causes a lot of awkwardness for the family later when they - and the rest of the tribe - figure out what the step-siblings are doing in secret. Also, Picknose's brother Dogsbreath and the girls' sister Inga are already married with a child by this point, but they live thousands of miles away, so are less involved with the rest of the family.
Literature
- My Stepmom's Daughter is My Ex: Exactly What It Says on the Tin. Mizuto and Yume used to date then broke up, and then their parents married each other.
- What Alice Forgot: One of the shocking realisations facing the amnesiac Alice is that, during the years she's forgotten, her father-in-law and her mother have gotten married.
Live-Action TV
- Awkwafina is Nora from Queens: Nora develops a mutual crush on Chuck, but after they learn that her father is dating Chuck's mother, the thought of becoming stepsiblings disgusts Nora and Chuck out of doing the same. Eventually, Chuck unsuccessfully tries to stop the parents from marrying by confessing his love for Nora.
- Brooklyn Nine-Nine: When Gina and Boyle break off their Friends with Benefits fling before Valentine's Day, they are horrified to realize they accidentally set their single parents up in the same romantic hotel room together, since each took their child's reservation. It gets more awkward when Boyle's father and Gina's mother decide to move in together. While the two initially try to break their parents up, they see how happy their parents are together, and even help them get married. Later subverted when Gina forces her cheating mother to divorce Boyle's dad. Boyle's tendency to use Accidental Innuendo does not help matters.
Boyle: Oh, it's just my former lover. Hello, sister.
Gina: Ugh. I hate when you say those things back-to-back. - Degrassi: Clare and Jake start secretly dating shortly after they meet as teens thanks to Clare getting over a rough breakup with Eli, but hit a snag when their parents begin dating and later get engaged. Initially Clare is concerned about it, but Jake convinces her to continue the relationship in secret. They last for several months until Clare proposes having sex with Jake, at which point he gets cold feet and they both realize they don't have long-term potential.
- Gilmore Girls: A variation, and zig-zagged. Luke and Lorelai are the overarching Will They or Won't They? couple of the series, with their attraction implied from the pilot. In season 2, Luke gets custody of his nephew, Jess, who is the same age Lorelai's daughter, Rory. Rory and Jess quickly form a Commonality Connection around books and briefly date. Luke and Lorelai are a Will They or Won't They? couple until almost the very end. Rory and Jess are Star-Crossed Lovers who can't get the timing right. These two love stories weave in and out both in the series proper and the revival—though there isn't a time in either series where both couples are dating at the same time. Subverted in that the relation is never discussed and it's a non-issue in-universe. Luke and Jess being nephew/uncle and Rory and Lorelai being mother/daughter actually plays heavily into the Like Parent, Like Spouse Love Triangle set up at the end of the revival. However, the fact that Luke and Lorelai marry at the end of the revival, making Jess and Rory step-cousins, was enough to make some newer fans jump ship.
- Gossip Girl zigzags this every which way with teens Serena and Dan and their parents Lily and Rufus. First Serena and Dan meet in their late teens and begin to date. Then Lily and Rufus marry. But also Lily and Rufus are Old Flames who dated — even had a kid together — back in the day before Serena and Dan were born. Serena and Dan are okay with the technically-stepsiblings-but-not-really-in-any-meaningful-sense thing, but having a shared half-sibling is a bit much for them. Then Lily and Rufus divorce. Then Dan and Serena get back together and end up marrying in the finale.
- How I Met Your Mother: Longtime couple Marshall and Lily are horrified when Marshall's mom starts hooking up with Lily's dad.
- Lucifer: Averted. When God decided to retire, his son Lucifer tried to set him up with his human girlfriend Chloe's mother. It didn't work because God was still in love with his ex (Lucifer's mother).
- Riverdale: Alice Smith and FP Jones retake their relationship, making Betty and Jughead's relationship this.
- Scream: The TV Series: Emma and Kieran have a Will They or Won't They? relationship for most of the first season, at the same time his father Clark and her mother Maggie, who are already Old Flames, dance around the same trope. Eventually they all realize they're dating around the same time, and end up accepting it and having a family dinner/double date. The only reason things don't work out is because Clark is killed by Ghostface, and in season 2 Kieran is revealed to be Ghostface himself.
- Sex Education has the unofficial variety of this. Otis and Ola date in Season 2, as does his mother, Jean, and her father, Jakob. However, both couples break up by the end of the season, but Jean accidentally becomes pregnant by Jakob. Come Season 3, all four are living in Jean's house, with her and Jakob resuming their relationship. Meanwhile, Otis and Ola must contend with their post-breakup hostility while facing the prospect of becoming stepsiblings.
- Sister Wives has a downplayed example in Kody and Janelle. They met as young adults and were friends. They introduced their parents, Winn and Sheryl, to each other. Their parents got married, and Kody and Janelle got married not long after.
Kody: It's not like Janelle was raised under my dad's roof, and it wasn't like I was raised under her mom's roof. We're not siblings. Gah, I just—I never even thought about how weird that was, cause you and I knew each other long before our parents knew each other.
Western Animation
- The Fairly OddParents!: Big Wanda has Mama Cosma (Cosmo's mom) and Big Daddy (Wanda's dad) fall in love with each other. Considering their respective children have been married for 9000 years, Cosmo is appropriately squicked out. In the end, it's subverted since the series doesn't show their relationship going past attraction.
- Silly Symphonies: In Music Land, at the end, the Saxophone King marries the Cello Queen and the Saxophone Prince marries the Violin Princess.
- The Simpsons: In "Lady Bouvier's Mother", Grandpa Simpson starts dating Marge's mother. Homer is opposed to this, because he thinks this would cause their kids to be inbred.
Marge: Homer, what possible reason could you have against Grandpa being in love?
Homer: If he marries your mother, Marge, we'll be brother and sister! And then our kids — they'll be horrible freaks with pink skin, no overbite, and FIVE FINGERS ON EACH HAND!! - 6teen: "The Journal" reveals that Jen had a crush on Jonesy back in the 7th grade, some years before Jonesy's dad and Jen's mom got married.
Real Life
- Lucy Schwob and Suzanne Malherbe — better known by their later pseudonyms Claude Cahun and Marcel Moore — met in 1909 at the ages of 15 and 17 and began a lifelong relationship. In 1917, their parents — the divorced Maurice Schwob and the widowed Marie Malherbe — married, making Cahun and Moore stepsisters after they'd already been in a relationship for 8 years. As a same-sex couple in that era, they presented themselves as sisters when they were living on the isle of Jersey during World War II, where they were part of an anti-Nazi resistance effort.
