Incense

From Thikipedia

Incense means having sex with family members and is a taboo in most cultures. Under British law, being incensed could lead to up to 12 years' imprisonment.

Sex with a second cousin is permitted and does not constitute incense. Sex with a first cousin is not permitted, unless they are really fit.

Sex with a step-brother or step-sister does not count as incense, as they are not blood relations. Sex with a half-brother or half-sister is permitted, as long as it's only the top half.