Basic Trope: BrotherSister Incest justified by one party being adopted—they're not biologically related.
- Straight: Alice is dating her adopted brother Bob.
- Alice dates her brother Bob, and then her 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
- 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.
- Subverted: Alice and Bob were Childhood Friends. Their mothers—Alice's mom Carol, and Bob's mom Dawn—were also dear friends. They had a lot in common, both being single mothers and having kids only one year apart in age, and they related to each other easily. When Dawn got sick, she asked Carol to take care of Bob, so Carol adopted him after Dawn died. Years later, after Alice and Bob had grown up and fallen in love, Alice's Disappeared Dad came back. He told Carol he was so sorry about abandoning her when she was pregnant. He tells them a story: He was young and irresponsible, and his girlfriend got pregnant. They were having problems before that, and when she got pregnant, he left her. He moved on, met Carol, and started dating her. And when she got pregnant too, he flipped out left because it was déjà vu. But he's matured since then, and he's really sorry. He wants to make amends, both with Carol and his ex-girlfriend from before her. Does Carol have any idea where he might find her—her name was Dawn.
- Double Subverted: The Disappeared Dad and Dawn were having problems just before she got pregnant and he left. These problems were mostly about Dawn cheating on him—he didn't believe the baby she was pregnant with was his. He wants to make amends now (he did leave in a jerkass way) and Bob could be his kid. Bob gets a Daddy DNA Test, which shows that he's not the his 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?
- Averted: Adopted siblings are treated the same as biological siblings in terms of incest.
- Enforced: The biological sibling incest in the original was bowdlerized .
- Lampshaded:Charlie: He's your brother, Alice!
Alice: He's adopted. Incest is inbreeding—this isn't inbreeding. Why shouldn't I be with him?
- Invoked: Alice's parents adopted Bob 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 don't act on their mutual attraction—they are siblings, and it would be wrong.
- Conversed: "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."
- Implied: Incest Subtext
- 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 Overprotective 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's children produced from the marriage, 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.
- Removing the genetic element allows for an interesting exploration of the other issues of incest.
- 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.