Basic Trope: Brother–Sister Incest justified by one or both parties being adopted—they're not biologically related.
- Straight: Alice is dating her adopted brother Bob.
- Exaggerated:
- Alice dates her adoptive brother Bob, and then her adoptive brother Charlie when she and Bob break up.
- Alice announces her adoption, and all of her brothers immediately begin fighting over who can have her despite seeing her as a sister up until that moment.
- Downplayed: Flirty Stepsiblings
- Justified:
- They were in a relationship before Bob was adopted.
- Childhood Friend Romance and First Girl Wins
- They became stepsibilings at a relatively late age such as adolescence or even adulthood, so they cannot be seen as siblings, since they did not grow up together.
- Inverted:
- Non-biological siblings being in a relationship is considered worse than biological siblings being in one—at least biological sibling incest can keep the bloodline pure.
- Learning they're not biologically related puts a cool on Alice and Bob's romantic relationship because they liked the thrill of taboo breaking.
- Subverted: Alice and Bob were childhood friends, and Alice's mother Carol adopted Bob after his mother Dawn died of an illness. When Alice and Bob grow up, they fall in love... but then, Alice's Disappeared Dad Edward comes back and reveals that he'd been with Dawn before Carol, and that Bob is Alice's half-brother.
- Double Subverted: Edward and Dawn where involved with each other but Dawn was cheating on him. Bob gets a Daddy DNA Test, which shows that he's not Edward's son, and thereby not Alice's brother.
- Parodied: Alice gets adopted by Bob's parents, dates Bob, gets disowned when she and Bob break up. Then she gets adopted by Charlie's parents, dates Charlie, and gets disowned when she and Charlie break up. Then, Alice gets adopted by Diane's parents, dates Diane, and gets disowned when they break up.
- Zig Zagged: Alice and her adopted brother Bob are caught in endless Will They or Won't They? along with numerous reveals about their true parentage.
- Averted:
- Adoptive siblings are treated the same as biological siblings in terms of incest.
- Alice and Bob were already completely fine with their sibling incest when they thought they were biologically related, and finding out they're not changes nothing.
- Enforced: The biological sibling incest in the original was bowdlerized.
- Lampshaded:Charlie: He's your brother, Alice!
Alice: He's adopted.
Charlie: But you grew up together!
Alice: I also grew up with Dan and no one minded when we dated. How is this different? - Invoked: Alice's parents adopted Bob specifically so Alice could marry him when they both grew up.
- Exploited: Bob loves stories about Little Sister Heroines, and Alice likes Bob. By constantly reminding him of their lack of blood ties, she hopes to appeal to his preferences—and eventually to his romantic affections.
- Defied: Adoptive siblings Alice and Bob refuse to act on their mutual attraction despite others pointing out they're not blood relatives—they are siblings, and it would be wrong.
- Discussed:Alice: As we get older, Bob is getting hot. I mean, I know he's kind of my brother, but he's adopted… dating him wouldn't be totally wrong, right?
- Conversed:Viewer 1: Why does everyone in books think adoption means kissing your siblings is totally ok? If you were raised together, the Westermarck Effect should still be in effect—it's still weird.
Viewer 2: ...you do know that the Westermarck Effect is a contested theory, and the evidence for it existing at all is shaky, right? - Implied: Incest Subtext
- Deconstructed:
- Dating Alice alienates Bob from his adoptive family—if he's not really related to Alice, he's not really related to any of his other family members either, is he? They're Alice's family, not his. Their dad now constantly grills Bob, treating him like every other boy who's ever dated Alice. And if Bob ever broke up with Alice, then what would happen? Their family would side with Alice, surely—Bob might even be disowned.
- Alice sees Bob as a man and treats him as such... but Bob doesn't or is trying not to. He just wants to get along as a family or may very we well see Alice as a sister, and their relationship becomes strained because of it.
- Dean and Ellie—Alice and Bob's parents' biological children—take a cue from their older not-blood-related siblings and start to fall for each other. When Alice and Bob try to stop them and say it's wrong, the new couple is confused, since that's what Alice and Bob did beforehand and they see it as normal to fall for your sibling. Alternatively, Bob and Alice's children do the same thing, and are confused for the same reason.
- Reconstructed:
- Removing the genetic element allows for an interesting exploration of the other issues of incest, such as the difference between familial and romantic love and whether they can coexist, or the fact that the same people consider both Alice and Bob their children.
- Alice and Bob teach their younger siblings that their case is a special case, and that not every sibling pair is going to end up like they did. They do acknowledge how hypocritical it seems coming from them, but they all now have to confront if their feelings make sense now that there is a biological component involved.
Back to Not Blood Siblings—it's a link, but not a biological link.
