Incest

Incest is romantic and especially sexual goings-on between relatives. It is illegal in most cultures in the multiverse. This does not stop fans, usually of The Movies, writing fanfic involving brothers or sisters having it off willingly, or parents abusing children. The PPC does not condone incest, and it is usually a charge. In cases where a Sue claims to be a sibling of a canon character, and yet still ends up having it off with them, this is still considered incest.

Twincest is a specialized kind of incest that takes place between twins. The most notorious victims are Fred and George Weasley and Elladan and Elrohir Peredhil.