Brother-Sister Incest/Playing With

Brother Sister Incest: A romantic, possibly sexual relationship between siblings, or half siblings related by blood.
 * Played Straight: Bob and Alice are very much in love and secretly hide their affection to avoid scorn from their peers.
 * Exaggerated: Bob and Alice are incredibly open about their relationship, and in fact, some of their friends/parents even encourage their relationship.
 * Downplayed:
 * There is an obvious romantic or sexual attraction between Bob and Alice, but neither one acts on it.
 * Incest Subtext
 * Justified:
 * Bob and Alice were in a relationship before they even knew they were related.
 * Alternatively, Bob and Alice are twins, and are considered to be born married by their culture.
 * Bob and Alice are villains that don't care about right or wrong (or are doing it precisely because it is "wrong").
 * Or they're creepy and unaware of the implications of Brother Sister Incest
 * Or one of them is an adopted sibling, and they think that since they aren't really related it's okay.
 * Bob and Alice are in place and time where incest isn't taboo, their parents may even encourage it to keep their bloodline pure.
 * Inverted:
 * Alice and Bob think that they're brother and sister but later find out that they aren't related, after which they become a couple.
 * Once the two siblings discover their blood ties they cut off every hint of romance between each other.
 * Bob and Alice react poorly to the suggestion by some of their friends they should do something that could be construed as sexual.
 * Or Alice cuts off the Romance but Bob still pines for her (Ala Enders Game).
 * Bob and Alice are trying to kill each other.
 * Subverted:
 * Alice and Bob decided that it would be for the best if they ended their relationship and saw other people because incest is 'unnatural.'
 * Not Blood Siblings
 * Doubly Subverted:
 * Alice and Bob are consumed with fantasies of their romance and end up coming back together again because their hearts already belong to each other.
 * Everyone thinks they're close because they're siblings. Except, it's Exactly What It Looks Like.
 * Zig Zagged: Their parents find out about their relationship and are horrified, separating Alice and Bob so they can never be together. During which time one or both of them painfully learn to let go of their love which apparently was not meant to be. Only to eventually meet later in life where their feelings are immediately rekindled.
 * Parodied: The siblings' parents make a wager to see how long it will take for Bob to get into Alice's pants.
 * Deconstructed:
 * Society conditionally won't accept Alice and Bob's love, driving both into a deep depression from the constant harassment they receive just because they're together. Ruining the happiness in their relationship.
 * Alice becomes pregnant, and the baby exhibits all the health problems associated with inbreeding, such as mental retardation and genetic diseases.
 * Or, alternatively, this is presented as completely normal.
 * Reconstructed:
 * Alice and Bob decided to move far away, where the society that couldn't accept them can no longer hurt them. And lead a happy life together.
 * The Doctor informs them that their baby is unhealthy because it was born prematurely, and that inbreeding had no factor.
 * The baby manages to overcome their short comings as they grow older, proving that they were not as unhealthy as feared.
 * Neither of them ever wanted to be a parent, so they just don't have children.
 * Or if they want to be parents, they decide to adopt.
 * Averted:
 * Bob does not have a sister.
 * Bob's sister dies a long time ago.
 * Bob does have a sister, but they have a normal relationship.
 * Enforced:
 * The author fantasizes about that.
 * The author is using incest as a plot point to create drama between Alice and Bob.
 * The author is using incest to make a point about society.
 * It's based-on-history work, about culture that Brother Sister Incest is common. It's nothing more than the author show that he did some research.
 * Lampshaded: "I know society will never accept us...but I don't care..."
 * Invoked:
 * Alice makes a childhood promise to Bob to marry him. Since Alice Will Not Tell a Lie, she fully plans to go through with her promise.
 * Alice was born so that she would marry Bob and continue the family bloodline.
 * Defied: Alice rejects Bob's advances.
 * Discussed: "Bob, this is wrong...We're brother and sister...nobody will ever understand..."
 * Conversed: "That's disgusting. I can't believe that sicko Bob is doing...that to his sister. His sister!"
 * Played for Laughs: Bob and Alice's parents grill him just like all of Alice's new boyfriends.

You can go back to Brother Sister Incest if you like, but...seriously, that's just wrong...