Basic Trope: Two characters are romantically involved with each other, only to discover they're actually related.
- Straight: Bob and Alice are in a relationship, but discover they are siblings.
- Exaggerated: Everyone Alice and Bob have been in relationship with turns out to be related to them.
- Downplayed:
- Bob and Alice flirted a bit, but didn't have a full-blown relationship before discovering they're siblings.
- Bob and Alice had a mutual crush on each other but they both moved on from it even before finding out they're related.
- Bob and Alice discover they are cousins. They live in a country where cousin marriage is legal, but heavily frowned upon.
- Justified:
- Bob and Alice were Separated at Birth.
- Bob and Alice are experiencing Genetic Sexual Attraction (a hypothesized phenomenon wherein which Long Lost Relatives who meet again years later are intensely attracted to one another).
- Inverted:
- Bob and Alice believe they're in an incestuous relationship, only to discover they're Not Blood Siblings.
- Subverted:
- Charlie tells Alice and Bob they're actually siblings… but he's lying.
- Bob's father sired Alice on the side with his mistress, except his mother also cheated, giving birth to Bob in the process.
- Double Subverted: The Man who Alice's mother slept with was the brother of her husband's mistress, so Alice and Bob are related after all.
- Parodied: Alice is in a relationship with Alicia, who looks and acts exactly like her. Both are shocked when they're told they're identical twins.
- Zig Zagged: Alice and Bob are dating and shortly thereafter, learn that Bob is adopted and Alice's parents are biologically Bob's as well, leading to their break-up. Bob then takes a DNA test to confirm this, which it seemingly does by revealing his biological relation to Alice's parents. But the doctor turns out to have a vendetta against Bob and wants Bob to think that he's dating his sister, causing him to switch Bob's results with Alice's brother Chuck — leaving the possibility that Bob isn't Alice's brother after all. This leads to the doctor's arrest and Bob getting another DNA test. This new test however reveals that Bob shares Alice's parents. Except Alice is revealed to be adopted thus making her parents her adoptive parents. Thus, Alice and Bob are adoptive siblings, but still siblings nonetheless.
- Averted: Bob and Alice do turn out to be related, but never have any romantic attraction to each other.
- Enforced: The executives believe Bob and Evelyn would make a more marketable couple, so they force the author to make Bob and Alice related so that Bob and Evelyn can get together.
- Lampshaded: "Oh my God, I can't believe I slept with my own sister! But how was I to know we were related?"
- Invoked: Professor Drachenstein deliberately misleads Bob and Alice since they're the only two Neo-Humans around for his Super Breeding Program.
- Exploited: Alice, wanting to get away from Domestic Abuser Bob, tricks him into thinking they're siblings.
- Defied: Alice suspects she and Bob might be related, so she refuses to enter a relationship with him to avoid this.
- Discussed:
- Conversed: "I feel so bad for Bob. What are the chances of your girlfriend turning out to be your own sister?"
- Implied:
- It's not clear if Bob and Alice were ever really in a relationship or if they were Just Friends, but they both look horrified upon discovering they're brother and sister (suggesting the former).
- Bob and Alice are a married couple who have suffered from a Tragic Stillbirth or the Inverse Law of Fertility. One day, Bob returns home from work, only to find Alice absent and some papers detailing both of their family histories. Bob reads the papers, and becomes shocked, but we never learn why exactly he reacted this way.
- Deconstructed: Alice and Bob are horrified when they learn they're brother and sister. They feel disgusted and confused about their feelings for each other and it eventually drives them apart, as they feel they can never have a normal sibling relationship with each other now.
- Played For Drama:
- Alice and Bob only discover they're siblings after learning why their youngest child is disabled. Freaked out by the revelation, not only are they awkward around each other but also their reasonably healthy children — letting irrationality destroy all of their positive connections.
- Alice and Bob were a case of Sex Equals Love, which brings them to feel disgusted with themselves.
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