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 do you call two siblings dating?

Sibling-in-law also refers to the reciprocal relationship between a persons spouse and their siblings spouse. In Indian English this can be referred to as a co-sibling (specificity a co-sister, for the wife of ones sibling-in-law, or co-brother, for the husband of ones sibling-in-law).