In what circumstances are Muggles allowed to know about magic?

In most cases, Muggles who witness magic or learn about it by other means have to have their memories removed. However, there are some exceptions:
1. Parents of Muggleborns (probably step-parents too)
2. Muggle guardians of any orphaned young wizards or witches
3. Muggle siblings
4. Any children of Muggle guardians, e.g. Dudley
5. Anyone who marries a witch or wizard
6. Current and past Muggle Prime Ministers of the UK (and probably their equivalents in other countries).

Can you think of any other situations where a Muggle would be allowed to know?