The most commonly used word would be incest, although technically, incest refers to sex between (close) relatives. However, since a marriage is usually supposed to be consummated (meaning the partners in the marriage do have sex), the word is also applied to marriage.

What is it called when siblings date?

While cousin marriage is legal in most countries, and avunculate marriage is legal in many, sexual relations between siblings are considered incestuous almost universally. Thus, many cases of sibling incest, including accidental incest, concern siblings who were separated at birth or at a very young age.

What is sibling romance called?

While the American Psychological Association does not use the term, GSA is what it sounds like: a phenomenon that occurs when two family members, who were separated early in life, eventually meet and experience an intense sexual attraction to each other — though not all act on it.