Aria and Ezra (Pretty Little Liars)
One of the earliest bits of evidence that this show was set on a different planet populated by beings that just happen to look like humans because that is the only way to explain how Ezra managed to avoid being charged with a felony after news got out that he was romantically involved with his student.
Clara and Ramona (The Bastard Executioner)
Working as prostitutes in a Welsh town, these two identical twin sisters don't mind engaging in a true "seeing double" threesome with their clients. And no, that isn't made possible by camera tricks, those are real-life identical twins making out passionately on screen, NAKED!
Norman & Norma Bates (Bates Motel)
The poster family for how not to raise your son and how much motherly love is too much. They do pretty much everything together, even cover up murders.
Ellen & Cavil (Battlestar Galactica)
Ellen built Cavil in the likeness of her dead father which technically made him her son who. Cavil, however, coerces her into sleeping with him in exchange for her husband's safety, though at the time she had no memory of her former life. Cavil, on the other hand, knew exactly who she was, but was more than happy to jump into those sexual encounters.
Clary and Jace (Shadowhunters)
A lot of people's favorite couple, but it's hard to shake the ick factor this pairing was smeared with when Valentine convinced everyone Jace was Clary's half-brother. They went from almost sleeping with each other, to familial love then back to sleeping with each other when the truth came out that Jace wasn't Jonathan. Just, NO!
Bill and Portia (True Blood)
In defense of these two, they had no idea they were related until Bill met Portia's grandmother who turned out to be Bill's great-great-great granddaughter. Bill does the honorable thing and tries to cut off all romantic ties between them but Portia is having none of that. She goes back to chasing Bill until he glamours her into screaming and running away in terror everytime she sees him.